test: define size_t to work around libclang oddities

For some reason clang 6 (and other versions) don't consider anything with
`size_t` to have comments unless we do this silly `typedef`.

We have a similar workaround in libgit2 itself.

Author: carlosmn

test/fixtures/git2/common.h

@@ -39,6 +39,12 @@
 # define GIT_END_DECL    /* empty */
 #endif
 
+/*
+ * libclang won't consider anything with size_t to have comments unless we use
+ * this hack.
+ */
+typedef size_t size_t;
+
 /** Declare a public function exported for application use. */
 #ifdef __GNUC__
 # define GIT_EXTERN(type) extern \