bpf: syscall_nrs: Disable no previous prototype warnning

In some environments (gcc treated as error in W=1, which is default), if we
make -C samples/bpf/, it will be stopped because of
"no previous prototype" error like this:

../samples/bpf/syscall_nrs.c:7:6:
error: no previous prototype for ‘syscall_defines’ [-Werror=missing-prototypes]
void syscall_defines(void)
^~~~~~~~~~~~~~~

Actually, this file meets our expectatations because it will be converted to
a .h file. In this way, it's correct. Considering the warnning stopping us
compiling, we can remove the warnning directly.

Signed-off-by: Jason Xing <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lore.kernel.org/all/CAEf4BzaVdr_0kQo=[e-mail]/
Link: https://lore.kernel.org/bpf/[e-mail]

samples/bpf/syscall_nrs.c

···
 #include <uapi/linux/unistd.h>
 #include <linux/kbuild.h>
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+
 #define SYSNR(_NR) DEFINE(SYS ## _NR, _NR)
 
 void syscall_defines(void)
···
 #endif
 
 }
+
+#pragma GCC diagnostic pop