Fix build on glibc >= 2.28

glibc 2.28 [removed](https://lwn.net/Articles/761462/) the `_G_config.h`
header, which defined the `_G_va_list` type. This wasn't intended to be
a public type. Replace it with `__gnuc_va_list` to fix compilation.

librevm/include/revm.h

@@ -419,7 +419,7 @@ extern int vsscanf(const char * restrict str, const char * restrict format,
  va_list ap);
 #elif defined(__linux__)
 extern int vsscanf (__const char *__restrict __s,
- __const char *__restrict __format, _G_va_list __arg);
+ __const char *__restrict __format, __gnuc_va_list __arg);
 #endif
 
 void wait4exit(void *);