examples/Makefile.am: changing header includes

The build of the examples is complaining about a function that are not
inline'd. The compiler is mentioned this because we use the '-I'
directive for pointing to the header files. Just say the compiler also
that the header files are system headers so the false positive is going
away.

examples/Makefile.am

@@ -9,7 +9,7 @@
 if BUILD_EXAMPLES
 
 # picking up the default warning CFLAGS into AM_CFLAGS
-AM_CFLAGS = -I@top_builddir@/include/coap/ $(WARNING_CFLAGS) -std=c99
+AM_CFLAGS = -isystem@top_builddir@/include/coap/ $(WARNING_CFLAGS) -std=c99
 
 # etsi_iot_01 and tiny are missing
 bin_PROGRAMS = coap-client coap-server rd