Add missing dependency for audio/rgain

I think audio/rgain is missing a runtime dependency. Running it after
install, I get:

/usr/local/bin/replaygain -h
Traceback (most recent call last):
File "/usr/local/bin/replaygain", line 3, in <module>
from rgain3.script.replaygain import replaygain
File "/usr/local/lib/python3.8/site-packages/rgain3/script/__init__.py", line 29, in <module>
import rgain3.rgio # noqa isort:skip
File "/usr/local/lib/python3.8/site-packages/rgain3/rgio.py", line 23, in <module>
import mutagen
ModuleNotFoundError: No module named 'mutagen'

Patch below fixes it. OKs?

Thanks,
Jeremy

Index: Makefile
===================================================================
RCS file: /cvs/ports/audio/rgain/Makefile,v
retrieving revision 1.8
diff -u -p -r1.8 Makefile
--- Makefile 3 Oct 2020 15:39:58 -0000 1.8
+++ Makefile 4 Oct 2020 05:21:00 -0000
@@ -5,7 +5,7 @@ COMMENT= multi-format tools and Python m
MODPY_EGG_VERSION= 1.0.0
DISTNAME= rgain3-${MODPY_EGG_VERSION}
EPOCH= 0
-REVISION= 0
+REVISION= 1

#WRKDIST= ${WRKDIR}/fk-rgain-8df1a846a12c

@@ -23,6 +23,7 @@ MODPY_VERSION= ${MODPY_DEFAULT_VERSION_3
BUILD_DEPENDS= textproc/py-docutils${MODPY_FLAVOR}

RUN_DEPENDS= devel/py-gobject3${MODPY_FLAVOR} \
+ audio/py-mutagen${MODPY_FLAVOR} \
multimedia/gstreamer1/core \
multimedia/gstreamer1/plugins-libav \
multimedia/gstreamer1/plugins-base \