Error out if we detect an incompatible libgit2 version

The master version supports only v0.24 so let's enforce that via the
compiler.
diff --git a/git.go b/git.go
index 45b01d5..ed891e6 100644
--- a/git.go
+++ b/git.go
@@ -4,6 +4,11 @@
 #include <git2.h>
 #include <git2/sys/openssl.h>
 #cgo pkg-config: libgit2
+
+#if LIBGIT2_VER_MAJOR != 0 || LIBGIT2_VER_MINOR != 24
+# error "Invalid libgit2 version; this git2go supports libgit2 v0.24"
+#endif
+
 */
 import "C"
 import (