[PATCH-for-7.0 v2] qga/vss-win32: fix compilation with clang++

[Helge Konetzka <hk@zapateado.de>]

This fixes:

qga/vss-win32/install.cpp:49:24: error: cannot initialize a variable of
type 'char *' with an rvalue of type 'const char *'
     char *msg = NULL, *nul = strchr(text, '(');
                        ^     ~~~~~~~~~~~~~~~~~

Signed-off-by: Helge Konetzka <hk@zapateado.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
Compiling with clang++ of msys2 toolchain clang64 leads to

[1445/1747] Compiling C++ object qga/vss-win32/qga-vss.dll.p/install.cpp.obj
FAILED: qga/vss-win32/qga-vss.dll.p/install.cpp.obj
...
qga/vss-win32/install.cpp:49:24: error: cannot initialize a variable of 
type 'char *' with an rvalue of type 'const char *'
     char *msg = NULL, *nul = strchr(text, '(');
                        ^     ~~~~~~~~~~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.
make: *** [Makefile:163: run-ninja] Error 1
==> ERROR: A failure occurred in build().
     Aborting...
---
  qga/vss-win32/install.cpp | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/qga/vss-win32/install.cpp b/qga/vss-win32/install.cpp
index 8076efe3cb..b57508fbe0 100644
--- a/qga/vss-win32/install.cpp
+++ b/qga/vss-win32/install.cpp
@@ -46,7 +46,8 @@ void errmsg(DWORD err, const char *text)
       * If text doesn't contains '(', negative precision is given, which is
       * treated as though it were missing.
       */
-    char *msg = NULL, *nul = strchr(text, '(');
+    char *msg = NULL;
+    const char *nul = strchr(text, '(');
      int len = nul ? nul - text : -1;

      FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
-- 
2.30.2