libbb: constify *bb_common_bufsiz1 (if it is compiled to be a pointer)

This lets gcc optimize much better:

   text	   data	    bss	    dec	    hex	filename
 922846	    910	  13056	 936812	  e4b6c	busybox_unstripped.nonconst
 920255	    910	  13056	 934221	  e414d	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

scripts/generate_BUFSIZ.sh

@@ -77,7 +77,7 @@ if test $REM -lt 1024; then
 	# users will need to malloc it.
 	{
 	echo "enum { COMMON_BUFSIZE = 1024 };"
-	echo "extern char *bb_common_bufsiz1;"
+	echo "extern char *const bb_common_bufsiz1;"
 	echo "void setup_common_bufsiz(void);"
 	} | regenerate "$common_bufsiz_h"
 	# Check that we aren't left with a buggy binary: