lib/uuid.c: change prototype of uuid_guid_get_str()

There's no reason to require an appropriately sized output parameter
for the string, that's error-prone should the table ever grow an
element with a longer string. We can just return the const char*
pointer directly.

Update the only caller accordingly, and get rid of pointless ifdeffery
in the header so that the compiler always sees a declaration and can
thus do type-checking, whether or not PARTITION_TYPE_GUID is enabled
or not.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/lib/uuid.c b/lib/uuid.c
index e62d5ca..6cfb6cd 100644
--- a/lib/uuid.c
+++ b/lib/uuid.c
@@ -122,20 +122,19 @@
  * uuid_guid_get_str() - this function get string for GUID.
  *
  * @param guid_bin - pointer to string with partition type guid [16B]
- * @param guid_str - pointer to allocated partition type string [7B]
+ *
+ * Returns NULL if the type GUID is not known.
  */
-int uuid_guid_get_str(const unsigned char *guid_bin, char *guid_str)
+const char *uuid_guid_get_str(const unsigned char *guid_bin)
 {
 	int i;
 
-	*guid_str = 0;
 	for (i = 0; i < ARRAY_SIZE(list_guid); i++) {
 		if (!memcmp(list_guid[i].guid.b, guid_bin, 16)) {
-			strcpy(guid_str, list_guid[i].string);
-			return 0;
+			return list_guid[i].string;
 		}
 	}
-	return -ENODEV;
+	return NULL;
 }
 #endif