fix(grc): source grc.zsh on gentoo (#12050)

Co-authored-by: mphi <mphi@norealmail.fdfe.de>
Co-authored-by: Carlo Sala <carlosalag@protonmail.com>

plugins/grc/grc.plugin.zsh

@@ -5,6 +5,7 @@ files=(
   /etc/grc.zsh               # default
   /usr/local/etc/grc.zsh     # homebrew darwin-x64
   /opt/homebrew/etc/grc.zsh  # homebrew darwin-arm64
+  /usr/share/grc/grc.zsh     # Gentoo Linux (app-misc/grc)
 )
 
 # verify the file is readable and source it