vma: avoid compiler warning about incompatible pointer type

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

debian/patches/pve/0027-PVE-Backup-add-vma-backup-format-code.patch

@@ -1735,7 +1735,7 @@ index 0000000000..ac7da237d0
 +}
 diff --git a/vma.c b/vma.c
 new file mode 100644
-index 0000000000..cf2a2a74af
+index 0000000000..347f6283ca
 --- /dev/null
 +++ b/vma.c
 @@ -0,0 +1,900 @@
@@ -1917,7 +1917,7 @@ index 0000000000..cf2a2a74af
 +    const char *filename;
 +    const char *dirname;
 +    const char *readmap = NULL;
-+    const gchar **drive_list = NULL;
++    gchar **drive_list = NULL;
 +
 +    for (;;) {
 +        c = getopt(argc, argv, "hvd:r:");