qga/vss-win32: fix compilation with clang++

qga/vss-win32: fix compilation with clang++

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>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-Id: <39400817-3dc9-516d-9096-bc1f68862531@zapateado.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>