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

Reported-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
No real risk here from a release PoV, it's just pulling a package
required for the Alpine clang build. Worse that cold happen is that
the Alpine clang build broke, but it's already broken.
---
 automation/build/alpine/3.12.dockerfile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/automation/build/alpine/3.12.dockerfile b/automation/build/alpine/3.12.dockerfile
index 2c02417ee6..94557e239b 100644
--- a/automation/build/alpine/3.12.dockerfile
+++ b/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 && \
-- 
2.30.1