[vimrc] Only load vim-easytags if ctags is installed

vimrc

@@ -103,12 +103,14 @@ if has("unix") || has("mac")
             \   'linter_errors': 'error'
             \ },
             \}
+        if executable('ctags')
             "Needed for vim-easytags"
             Plugin 'xolox/vim-misc'
             Plugin 'xolox/vim-easytags'
             let g:easytags_async=1
             "Needed for YouCompleteMe"
             let g:easytags_opts=['--fields=+l']
+        endif
         Plugin 'godlygeek/tabular'
         Plugin 'beyondmarc/opengl.vim'
         Plugin 'tikhomirov/vim-glsl'