rilmodem: fix compilation on gcc-6.3

gcc-6 fails to compile ofono due to #pragma directives. Limit them to
gcc versions where they are needed.

drivers/rilmodem/call-forwarding.c

@@ -37,7 +37,9 @@
 #include <ofono/call-forwarding.h>
 #include "common.h"
 
+#if __GNUC__ > 7
 #pragma GCC diagnostic ignored "-Wrestrict"
+#endif
 
 #include "gril.h"
 

drivers/rilmodem/network-registration.c

@@ -36,7 +36,9 @@
 #include <ofono/modem.h>
 #include <ofono/netreg.h>
 
+#if __GNUC__ > 7
 #pragma GCC diagnostic ignored "-Wrestrict"
+#endif
 
 #include <gril/gril.h>