Fix Sparse warning about missing prototype

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/bsd-user/elfload.c b/bsd-user/elfload.c
index 678f3e6..48ec4ac 100644
--- a/bsd-user/elfload.c
+++ b/bsd-user/elfload.c
@@ -545,8 +545,6 @@
         memcpy(to, from, n);
 }
 
-extern unsigned long x86_stack_size;
-
 static int load_aout_interp(void * exptr, int interp_fd);
 
 #ifdef BSWAP_NEEDED
diff --git a/bsd-user/qemu.h b/bsd-user/qemu.h
index 822a214..d375611 100644
--- a/bsd-user/qemu.h
+++ b/bsd-user/qemu.h
@@ -193,6 +193,9 @@
 void mmap_fork_end(int child);
 #endif
 
+/* main.c */
+extern unsigned long x86_stack_size;
+
 /* user access */
 
 #define VERIFY_READ 0