[build] Avoid sparse undeclared symbol warning for PROVIDE_SYMBOL()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/compiler.h

 	.comm	_sym, 0
 #else /* ASSEMBLY */
 #define PROVIDE_SYMBOL( _sym )				\
+	extern char _sym[];				\
 	char _sym[0]
 #endif /* ASSEMBLY */