Modified addition of libgit2 to JL_PRIVATE_LIBS

Keeping the "lib" suffix in libgit2 breaks this line

 line 176: cp -a $(BUILD)/$(JL_LIBDIR)/lib$${suffix}*.$(SHLIB_EXT)*
 $(PREFIX)/$(JL_PRIVATE_LIBDIR) ;

which results in build system errors.  This commit fixes that.

Ref #4158