gcc-source: Fix gengtypes race

gcc renamed .c files to .cc files:

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=5c69acb32329d49e58c26fa41ae74229a52b9106



but we didn't fix this reference which meant we re-introduced a race around
gengtypes-lex.c. This lead to the race reappearing on the autobuilder. Fix
the naming to avoid the problem again.

[YOCTO #14953]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>