bsd-user: style tweak: move extern to header file

extern char **environ has no standard home, so move the declaration from the .c
file to a handy .h file. Since this is a standard, old-school UNIX interface
dating from the 5th edition, it's not quite the same issue that the rule is
supposed to protect against, though.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>

bsd-user/main.c

@@ -47,7 +47,6 @@ unsigned long reserved_va;
 
 static const char *interp_prefix = CONFIG_QEMU_INTERP_PREFIX;
 const char *qemu_uname_release;
-extern char **environ;
 enum BSDType bsd_type;
 
 /*

bsd-user/qemu.h

@@ -27,6 +27,8 @@
 
 #include "exec/user/abitypes.h"
 
+extern char **environ;
+
 enum BSDType {
     target_freebsd,
     target_netbsd,