Incorrect target file name for gramLatex.c

Hi!

The attached patch changes the rule that describes the actions for
gramLatex.c in src/library/tools/src/Makefile.in so it actually
generates that file instead of "gramLatex." (no extension). The
file name without extension is not referenced anywhere else and
in R-2.12 the same rule still used the full name, so it appears
that the "c" was lost in editing somewhere along the way.

The patch was generated for R-3.0.1, but also applies cleanly to
R-devel_2013-05-14.