kbuild: fix C libary confusion in unifdef.c due to getline()

This fixes an error when compiling the kernel.

  CHK     include/linux/version.h
  HOSTCC  scripts/unifdef
scripts/unifdef.c:209: error: conflicting types for 'getline'
/usr/include/stdio.h:651: note: previous declaration of 'getline' was here
make[1]: *** [scripts/unifdef] Error 1
make: *** [__headers] Error 2

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Cc:  Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>