windows: Don't declare [v]asprintf()

None of our build environments seem to require these declarations.  And
current versions of MinGW-w64 define them as inline functions in stdio.h
so these declarations clashed with that ("static declaration of '...'
follows non-static declaration").

src/libstrongswan/utils/compat/windows.h

@@ -134,16 +134,6 @@ const char WINAPI *inet_ntop(int af, const void *src, char *dst, socklen_t size)
 int WINAPI inet_pton(int af, const char *src, void *dst);
 #endif
 
-/**
- * Provided by printf hook backend
- */
-int asprintf(char **strp, const char *fmt, ...);
-
-/**
- * Provided by printf hook backend
- */
-int vasprintf(char **strp, const char *fmt, va_list ap);
-
 /**
  * timeradd(3) from <sys/time.h>
  */