git-compat-util: move strbuf.c funcs to its header

While functions like starts_with() probably should not belong in the
boundaries of the strbuf library, this commit focuses on first splitting
out headers from git-compat-util.h.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/unpack-objects.c

@@ -12,6 +12,7 @@
 #include "blob.h"
 #include "commit.h"
 #include "replace-object.h"
+#include "strbuf.h"
 #include "tag.h"
 #include "tree.h"
 #include "tree-walk.h"