ref-manual: Added "leakage" FAQ entry.

Fixes [YOCTO #6852]

I added a new FAQ entry to address situations where during a
native build, the build fails due to previously installed stuff
on the system from /usr/local/include and /opt.  The entry
tries to cover "leakage" issues in general but calls out the
specific case involving libiconv and iconv.h.

(From yocto-docs rev: 2c09c7ee0970189953bfbbdcaac49a1ebe8482fe)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/faq.xml

@@ -361,6 +361,41 @@
         </answer>
     </qandaentry>
 
+    <qandaentry>
+        <question>
+            <para>
+                When I try to build a native recipe, the build fails with <filename>iconv.h</filename> problems.
+            </para>
+        </question>
+        <answer>
+            <para>
+                If you get an error message that indicates GNU
+                <filename>libiconv</filename> is not in use but
+                <filename>iconv.h</filename> has been included from
+                <filename>libiconv</filename>, you need to check to see if
+                you have a previously installed version of the header file
+                in <filename>/usr/local/include</filename>.
+                <literallayout class='monospaced'>
+     #error GNU libiconv not in use but included iconv.h is from libiconv
+                </literallayout>
+                If you find a previously installed file, you should either
+                uninstall it or temporarily rename it and try the build again.
+            </para>
+
+            <para>
+                This issue is just a single manifestation of "system
+                leakage" issues caused when the OpenEbedded build system
+                finds and uses previously installed files during a native
+                build.
+                This type of issue might not be limited to
+                <filename>iconv.h</filename>.
+                Be sure that leakage cannot occur from
+                <filename>/usr/local/include</filename> and
+                <filename>/opt</filename> locations.
+            </para>
+        </answer>
+    </qandaentry>
+
     <qandaentry>
         <question>
             <para>