bsd-user: fix compile failure

bsd-user doesn't actually support reserving a memory area for the
guest address space, but we need to at least define the reserved_va
global so that cpu-all.h's RESERVED_VA macro will work correctly.

This fixes a compilation error introduced in commit 39879bb
which added a use of RESERVED_VA to h2g_valid().

Reported-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

bsd-user/main.c

@@ -41,6 +41,7 @@ int singlestep;
 unsigned long mmap_min_addr;
 unsigned long guest_base;
 int have_guest_base;
+unsigned long reserved_va;
 #endif
 
 static const char *interp_prefix = CONFIG_QEMU_INTERP_PREFIX;