neovim: add gettext-dev

Fixes:

    [ 50%] Building C object src/nvim/CMakeFiles/nvim.dir/eval.c.o
    /vim/neovim-neovim/neovim-master/src/nvim/eval.c: In function 'f_foldtext':
    /vim/neovim-neovim/neovim-master/src/nvim/eval.c:8717:11: warning: implicit declaration of function 'ngettext' [-Wimplicit-function-declaration]
         txt = ngettext("+-%s%3ld line: ", "+-%s%3ld lines: ", count);
               ^~~~~~~~
    /vim/neovim-neovim/neovim-master/src/nvim/eval.c:8717:9: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
         txt = ngettext("+-%s%3ld line: ", "+-%s%3ld lines: ", count);
             ^

scripts/install_vim.sh

@@ -142,7 +142,8 @@ EOF
     # Some of them will be installed already, but it is a good reference for
     # what is required.
     # luajit is required with Neomvim 0.2.1+ (previously only during build).
-    apk add libuv \
+    apk add gettext \
+      libuv \
       libtermkey \
       libvterm \
       luajit \
@@ -154,6 +155,7 @@ EOF
       ca-certificates \
       cmake \
       g++ \
+      gettext-dev \
       gperf \
       libtool \
       libuv-dev \