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

[Helge Konetzka]

Compiling with clang++ of msys2 toolchain clang64 leads to an error
on initializing 'char*' variable nul with a 'const char*' value

Signed-off-by: Helge Konetzka <hk@zapateado.de>
---
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