Fix -Wimplicit-function-declaration compilation warning by moving libgeom.h

#include below the stdio.h #include.

gctl_dump(3) needs stdio.h, per reasoning noted in r317289.

MFC after:	5 weeks
PR:		218809
Submitted by:	Chang-Hsien Tsai <luke.tw@gmail.com>
Sponsored by:	Dell EMC Isilon

tools/regression/geom_gpt/test.c

@@ -29,12 +29,12 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <errno.h>
-#include <libgeom.h>
 #include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <libgeom.h>
 
 struct retval {
 	struct retval *retval;