um: fix strrchr() problems

richard@nod.at:
Fixes:
  /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/libc.a(strrchr.o): In function `rindex':
  (.text+0x0): multiple definition of `strrchr'

If both STATIC_LINK and UML_NET_VDE are set to "y" libc's strrchr may
clash with the kernel implementation.

This workaround comes originally from Jeff Dike:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=494995#35

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>