fix compilation on FreeBSD (make the -lgit2 link flag universal)

Author: spytheman

git/libgit.v

@@ -7,8 +7,7 @@ module git
 #flag darwin -I/opt/homebrew/include
 #flag darwin -L/opt/homebrew/lib
 
-#flag darwin -lgit2
-#flag linux -lgit2
+#flag -lgit2
 
 #include "git2/types.h"
 #include "git2/common.h"
@@ -46,6 +45,9 @@ struct C.git_error {
 	message &char
 }
 
+[typedef]
+struct C.git_tree_entry {}
+
 fn C.git_commit_message(voidptr) &char
 fn C.git_reference_lookup(&&C.git_reference, &C.git_repository, &char)
 fn C.git_reference_symbolic_target(&C.git_reference) &char