test: fix -Wsign-compare in proftest _mkdir_recursive loop

The functional-test harness compared a signed loop index against strlen()'s
size_t, which the newly promoted -Wsign-compare turns into a -Werror failure
when the test binaries are built. Make the index size_t.

Refs #112

tests/functionaltests/proftest.c

@@ -119,7 +119,7 @@ _create_dir(const char* name)
 gboolean
 _mkdir_recursive(const char* dir)
 {
-    int i;
+    size_t i;
     gboolean result = TRUE;
 
     for (i = 1; i <= strlen(dir); i++) {