git-compat-util: avoid redeclaring _DEFAULT_SOURCE

We are about to vendor in `mimalloc`'s source code which we will want to
include `git-compat-util.h` after defining that constant.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

git-compat-util.h

@@ -185,7 +185,9 @@ struct strbuf;
 #define _ALL_SOURCE 1
 #define _GNU_SOURCE 1
 #define _BSD_SOURCE 1
+#ifndef _DEFAULT_SOURCE
 #define _DEFAULT_SOURCE 1
+#endif
 #define _NETBSD_SOURCE 1
 #define _SGI_SOURCE 1