eal: ignore gcc-compat warning in clang-only macro

eal: ignore gcc-compat warning in clang-only macro

When compiling with clang using -Wpedantic (or -Wgcc-compat) the use of
diagnose_if kicks up a warning:

.../include/rte_interrupts.h:623:1: error: 'diagnose_if' is a clang
extension [-Werror,-Wgcc-compat]
__rte_internal
^
.../include/rte_compat.h:36:16: note: expanded from macro '__rte_internal'
__attribute__((diagnose_if(1, "Symbol is not public ABI", "error"), \

This change ignores the '-Wgcc-compat' warning in the specific location
where the warning occurs.  It is safe to do in this circumstance as the
specific macro is only defined when using the clang compiler.

Signed-off-by: Michael Barker <mikeb01@gmail.com>