vim: ensure configure doesn't fail if X11 packages are enabled

When X11 is detected, vim's configure does a AC_TRY_RUN to detect wchar_t
size, so preset it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/vim/vim.mk

@@ -17,7 +17,9 @@ VIM_CONF_ENV = vim_cv_toupper_broken=no \
 		vim_cv_getcwd_broken=no \
 		vim_cv_stat_ignores_slash=yes \
 		vim_cv_memmove_handles_overlap=yes \
-		ac_cv_sizeof_int=4
+		ac_cv_sizeof_int=4 \
+		ac_cv_small_wchar_t=no
+
 VIM_CONF_OPT = --with-tlib=ncurses
 
 define VIM_INSTALL_TARGET_CMDS