Update git_dynamic.go

this branch should support libgit2 v0.26.0
diff --git a/git_dynamic.go b/git_dynamic.go
index f89d509..00828a5 100644
--- a/git_dynamic.go
+++ b/git_dynamic.go
@@ -6,8 +6,8 @@
 #include <git2.h>
 #cgo pkg-config: libgit2
 
-#if LIBGIT2_VER_MAJOR != 0 || LIBGIT2_VER_MINOR != 25
-# error "Invalid libgit2 version; this git2go supports libgit2 v0.25"
+#if LIBGIT2_VER_MAJOR != 0 || LIBGIT2_VER_MINOR != 26
+# error "Invalid libgit2 version; this git2go supports libgit2 v0.26"
 #endif
 
 */