include sys/param.h in getcwd.c

[Joseph S. Myers]

I came across this 2006 patch 
<http://sourceware.org/ml/libc-alpha/2006-06/msg00068.html> in the course 
of reviewing bugs.  I think it should go in as an obvious fix - source 
files should use well-defined APIs of the headers they include rather than 
relying on long indirect include paths for internal headers.  Comments?

-- 
Joseph S. Myers
joseph@codesourcery.com