nls-global.mk: search build dir for source files, too

In VPATH builds, the build directory was not being searched for files in
GETTEXT_FILES, leading to failure to construct the .pot files.  This has
bit me all along, but never hard enough to get it fixed; I suppose not a
lot of people uses VPATH and NLS-enabled builds, and those that do,
don't do "make update-po" often.

This is a longstanding problem, so backpatch all the way back.