Hidden dependency for print/ptouch-driver

Had a failure of print/ptouch-driver on the current sparc64 build.

ptexplain.c: In function 'main':
ptexplain.c:813: error: 'HAVE_LIBPNG' undeclared (first use in this function)
ptexplain.c:813: error: (Each undeclared identifier is reported only once
ptexplain.c:813: error: for each function it appears in.)
gmake[1]: *** [Makefile:531: ptexplain.o] Error 1

Adding graphics/png to LIB_DEPENDS fixes it.

ok?

(cc maintainer)

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/print/ptouch-driver/Makefile,v
retrieving revision 1.10
diff -u -p -r1.10 Makefile
--- Makefile 28 May 2020 08:47:38 -0000 1.10
+++ Makefile 2 Aug 2020 17:04:13 -0000
@@ -6,6 +6,7 @@ GH_TAGNAME= v1.5.1
GH_ACCOUNT= philpem
GH_PROJECT= printer-driver-ptouch
DISTNAME= ptouch-driver-${GH_TAGNAME:S/v//}
+REVISION= 0

WRKDIST= ${WRKDIR}/ptouch-driver-${GH_TAGNAME:S/v//}

@@ -22,7 +23,8 @@ MASTER_SITES= https://github.com/philpe

BUILD_DEPENDS= textproc/p5-XML-LibXML

-LIB_DEPENDS= print/cups,-libs
+LIB_DEPENDS= graphics/png \
+ print/cups,-libs

# foomatic-rip(1), foomatic-db{,-engine}
RUN_DEPENDS += print/cups-filters