flex: disable documentation

Since we prevent the flex binary to be built, we also need to prevent
the documentation to be built, otherwise it will fail like this:

Making all in doc
make[2]: Entering directory '/br/output/build/flex-2.6.4/doc'
make[2]: *** No rule to make target '../src/flex', needed by 'flex.1'.
Stop.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>

diff -rup a/Makefile.in b/Makefile.in
--- a/Makefile.in	2017-05-06 15:49:09.000000000 +0100
+++ b/Makefile.in	2017-05-10 14:35:20.152794902 +0100
@@ -450,7 +450,6 @@ EXTRA_DIST = \
 
 SUBDIRS = \
 	src \
-	doc \
 	examples \
 	po \
 	tests \
