include/compiler.h: remove redundant declaration of errno

Commit 37566090 "compiler.h: unify system ifdef cruft here" added both
a "#include <errno.h>" and a "extern int errno;" to include/compiler.h
which is causing build warnings for some systems, for example for the
"netstar" board:

	In file included from /home/wd/git/u-boot/work/lib/crc32.c:15:
	include/compiler.h:28: warning: function declaration isn't a prototype

The declaration of "errno" should be redundant, as <errno.h> is
supposed to provide a correct declaration, so drop it.
Signed-off-by: NWolfgang Denk <wd@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>

include/compiler.h

@@ -25,8 +25,6 @@
 #include <stdio.h>
 #include <string.h>
 
-extern int errno;
-
 #if !defined(__WIN32__) && !defined(__MINGW32__)
 # include <sys/mman.h>
 #endif