Add optdepend for battery api (FS#42781)

PKGBUILD

@@ -16,7 +16,8 @@ makedepends=('unzip' 'zip' 'diffutils' 'python2' 'yasm' 'mesa' 'imake'
              'inetutils')
 optdepends=('networkmanager: Location detection via available WiFi networks'
             'gst-plugins-good: h.264 video'
-            'gst-libav: h.264 video')
+            'gst-libav: h.264 video'
+            'upower: Battery API')
 install=firefox.install
 options=('!emptydirs' '!makeflags')
 source=(https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/$pkgver/source/firefox-$pkgver.source.tar.bz2{,.asc}