[mom]: Fix example build dependencies

We are seeing occasional failures for eqn being missing when building
mom examples, e.g.:

| groff-1.24.0/groff: error: couldn't exec eqn: Permission denied
| pdfmom: fatal error: groff-1.24.0/test-groff exited with status 24
| make[2]: *** [Makefile:19256: contrib/mom/examples/slide-demo.pdf] Error 1

It may find eqn from the host system if installed too.

Fix the build dependencies to add eqn, pic, tbl and preconv since a 
quick strace indicates them all being used here

Upstream-Status: Backport [https://cgit.git.savannah.gnu.org/cgit/groff.git/commit/?id=3f7d8ea20f91a8abf248fbf7ec73049cd8c3c9c2]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Index: groff-1.24.0/contrib/mom/mom.am
===================================================================
--- groff-1.24.0.orig/contrib/mom/mom.am
+++ groff-1.24.0/contrib/mom/mom.am
@@ -144,8 +144,8 @@ SUFFIXES += .mom .pdf
 	&& LC_ALL=C $(MOMPDFMOM) $< >$@
 
 $(MOMPROCESSEDEXAMPLEFILES): $(MOMNORMALFILES) \
-  test-groff groff troff gropdf pdfmom penguin.ps penguin.pdf \
-  font/devpdf/stamp tmac/pdf.tmac
+  test-groff groff troff gropdf pdfmom eqn pic tbl preconv \
+  penguin.ps penguin.pdf font/devpdf/stamp tmac/pdf.tmac
 
 penguin.ps:
 	$(AM_V_at)cp $(mom_srcdir)/examples/penguin.ps $@