fix: include git2/sys/alloc.h for git_allocator on libgit2 1.8+

git_allocator moved out of the top-level git2.h into git2/sys/alloc.h
in libgit2 1.8.0. Add an explicit include so the mimalloc binding
compiles against libgit2 >= 1.8 (e.g. MacPorts libgit2 1.9.4).

Author: eejd

internal/cbm/cbm.c

@@ -20,7 +20,9 @@
 #if defined(CBM_BIND_TS_ALLOCATOR) && CBM_BIND_TS_ALLOCATOR
 #include "sqlite3.h" // sqlite3_mem_methods, sqlite3_config, SQLITE_CONFIG_MALLOC — bind sqlite to mimalloc
 #if defined(HAVE_LIBGIT2)
-#include <git2.h> // git_allocator, git_libgit2_opts, GIT_OPT_SET_ALLOCATOR — bind libgit2 to mimalloc
+#include <git2.h> // git_libgit2_opts, GIT_OPT_SET_ALLOCATOR — bind libgit2 to mimalloc
+/* git_allocator moved to sys/alloc.h in libgit2 1.8+; no longer in git2.h */
+#include <git2/sys/alloc.h>
 #endif
 #endif
 #include <stdint.h> // uint32_t, uint64_t, int64_t