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

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Amos Kong <akong@redhat.com>
diff --git a/vl.c b/vl.c
index 7511e70..ed8debb 100644
--- a/vl.c
+++ b/vl.c
@@ -230,7 +230,7 @@
 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];