fix iconv headers discovery

Author: mvorisek

.github/actions/configure-x64/action.yml

@@ -41,6 +41,7 @@ runs:
           --enable-pcntl \
           --with-readline \
           --enable-mbstring \
+          $(if [ -f /etc/alpine-release ]; then echo --with-iconv=/usr/include/gnu-libiconv; else echo ; fi) \
           --with-curl \
           --with-gettext \
           --enable-sockets \