build: fix maybe-uninitialized error on some gcc versions

on gcc (GCC) 7.4.0

devtools/create-gossipstore.c: In function ‘main’:
devtools/create-gossipstore.c:107:9: error: ‘infd’ may be used uninitialized ..
  while (read_all(infd, &be_inlen, sizeof(be_inlen))) {
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: William Casarin <jb55@jb55.com>

devtools/create-gossipstore.c

@@ -37,7 +37,7 @@ int main(int argc, char *argv[])
 	struct amount_sat sat;
 	bool verbose = false;
 	char *infile = NULL, *outfile = NULL, *scidfile = NULL, *csat = NULL;
-	int infd, outfd;
+	int infd = 0, outfd;
        	FILE * scidfd;
 	struct scidsat * scids = NULL;
 	unsigned max = -1U;