multipath-tools: fix an assignment ambiguity

spotted by indent:
./kpartx/gpt.c:623: Warning:old style assignment ambiguity in "=-". Assuming "= -"

Cc: Martin Wilck <mwilck@suse.com>
Cc: Benjamin Marzinski <bmarzins@redhat.com>
Cc: Christophe Varoqui <christophe.varoqui@opensvc.com>
Cc: DM-DEVEL ML <dm-devel@lists.linux.dev>
Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>

kpartx/gpt.c

@@ -606,7 +606,7 @@ read_gpt_pt (int fd, __attribute__((unused)) struct slice all,
 	gpt_entry *ptes = NULL;
 	unsigned int i;
 	int n = 0;
-	int last_used_index=-1;
+	int last_used_index = -1;
 	int sector_size_mul = get_sector_size(fd)/512;
 
 	if (!find_valid_gpt (fd, &gpt, &ptes) || !gpt || !ptes) {