Don't mark libgit2 target as phony, don't rebuild it each time

If the dependency for libegit2 is phony libegit2 is rebuild each time
make is ran despite it not having changed.

Signed-off-by: Björn Bidar <bjorn.bidar@thaodan.de>

Makefile

@@ -62,7 +62,7 @@ EMACS_ARGS ?=
 
 LOAD_PATH  ?= -L . -L build
 
-.PHONY: test libgit2 submodule-update install
+.PHONY: test submodule-update install
 
 all: lisp