Fixes a typo in Makefile.comm

Upstream fix:
http://lists.gnu.org/archive/html/bug-groff/2011-01/msg00002.html

diff -ur groff-1.21.old/Makefile.comm groff-1.21/Makefile.comm
--- groff-1.21.old/Makefile.comm	2010-12-31 08:33:08.000000000 +0100
+++ groff-1.21/Makefile.comm	2011-01-21 16:19:59.762069900 +0100
@@ -21,7 +21,7 @@
 #
 INCLUDES=-I. -I$(srcdir) \
   -I$(top_builddir)/src/include -I$(top_srcdir)/src/include \
-  -I$(tob_builddir)/src/libs/gnulib/lib
+  -I$(top_builddir)/src/libs/gnulib/lib
 ALL_CCFLAGS=$(INCLUDES) $(CCDEFINES) $(CCFLAGS) $(CPPFLAGS)
 COMPILE.cpp=$(CCC) $(ALL_CCFLAGS) -c
 ALL_CFLAGS=$(INCLUDES) $(CDEFINES) $(CFLAGS) $(CPPFLAGS)