Repeat execvpe(3) prototype (fup to 7bad9d7d)

This is needed in case `<unistd.h>` was included before "execvpe.h"
w/o `_GNU_SOURCE` set (on Glibc systems)

include/execvpe.h

@@ -16,6 +16,8 @@ __hsunix_execvpe(const char *name, char *const argv[], char *const envp[]);
 #if HAVE_EXECVPE
 # define _GNU_SOURCE
 # include <unistd.h>
+extern int
+execvpe(const char *name, char *const argv[], char *const envp[]);
 #else
 # define execvpe(name,argv,envp) __hsunix_execvpe(name,argv,envp)
 #endif