t0101-oid.c: Fix a memory leak reported by valgrind

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>

tests/t0101-oid.c

 #include "test_lib.h"
 #include <git/oid.h>
+#include <stdlib.h>
 
 BEGIN_TEST(oid_szs)
 	git_oid out;
@@ -186,6 +187,7 @@ BEGIN_TEST(cmp_oid_allocfmt)
 	out = git_oid_allocfmt(&in);
 	must_be_true(out);
 	must_pass(strcmp(exp, out));
+	free(out);
 END_TEST
 
 BEGIN_TEST(cmp_oid_pathfmt)