automation/alpine: add g++ to the list of build depends

clang++ relies on the C++ headers installed by g++, or else a clang
build will hit the following error:

<built-in>:3:10: fatal error: 'cstring' file not found
#include "cstring"
         ^~~~~~~~~
1 error generated.
make[10]: *** [Makefile:120: headers++.chk] Error 1

Link: https://gitlab.alpinelinux.org/alpine/aports/-/issues/12492
Reported-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

automation/build/alpine/3.12.dockerfile

@@ -21,6 +21,7 @@ RUN \
   apk add curl-dev && \
   apk add dev86 && \
   apk add gcc  && \
+  apk add g++ && \
   apk add clang  && \
   apk add gettext && \
   apk add git && \