selftests: vdso: Use a header file to prototype parse_vdso API

Both vdso_test_gettimeofday and vdso_standalone_test_x86 use the library in
parse_vdso.c but each separately declares the API it offers which is not
ideal. Create a header file with prototypes of the functions and use it in
both the library and the tests to ensure that the same prototypes are used
throughout.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/vDSO/vdso_test_gettimeofday.c

@@ -17,10 +17,7 @@
 #include <sys/time.h>
 
 #include "../kselftest.h"
-
-extern void *vdso_sym(const char *version, const char *name);
-extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
-extern void vdso_init_from_auxv(void *auxv);
+#include "parse_vdso.h"
 
 /*
  * ARM64's vDSO exports its gettimeofday() implementation with a different