vl: make boot_strict variable static (not used outside vl.c)

Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>
Reviewed-by: NAmos Kong <akong@redhat.com>

vl.c

@@ -230,7 +230,7 @@ int ctrl_grab = 0;
 unsigned int nb_prom_envs = 0;
 const char *prom_envs[MAX_PROM_ENVS];
 int boot_menu;
-bool boot_strict;
+static bool boot_strict;
 uint8_t *boot_splash_filedata;
 size_t boot_splash_filedata_size;
 uint8_t qemu_extra_params_fw[2];