po-docs: Make sure guestmount.1 depends on includes

guestmount.1 depends on translated files blocksize-option.pod,
key-option.pod & keys-from-stdin-option.pod (via __INCLUDE__
directives).  If these are not yet translated by the time we try to
generate guestmount.1 then it will fail with:

podwrapper.pl: key-option.pod: cannot find input file on path at /builddir/build/BUILD/libguestfs-1.50.1/podwrapper.pl line 672.

This happens especially in parallel builds.  Fix this by writing the
guestmount.1 rule explicitly, with the correct dependencies.

(cherry picked from commit 87b4d19f85)
(cherry picked from commit 17061cf496)

po-docs/language.mk

@@ -110,6 +110,13 @@ guestfish.1: guestfish.pod guestfish-actions.pod guestfish-commands.pod guestfis
 	  --license GPLv2+ \
 	  $<
 
+guestmount.1: guestmount.pod blocksize-option.pod key-option.pod keys-from-stdin-option.pod
+	$(PODWRAPPER) \
+	  --no-strict-checks \
+	  --man $@ \
+	  --license GPLv2+ \
+	  $<
+
 virt-builder.1: virt-builder.pod customize-synopsis.pod customize-options.pod
 	$(PODWRAPPER) \
 	  --no-strict-checks \