Adds missing unsupported_browser.css.

Makefile

 UGLIFYJS = ./node_modules/.bin/uglifyjs
 EXORCIST = ./node_modules/.bin/exorcist
 CLEANCSS = ./node_modules/.bin/cleancss
-CSS_FILES = chat.css contact_list.css feedback.css font-awesome.css font.css jitsi_popover.css jquery-impromptu.css login_menu.css main.css modaldialog.css notice.css popover.css popup_menu.css settingsmenu.css toastr.css videolayout_default.css welcome_page.css
+CSS_FILES = chat.css contact_list.css feedback.css font-awesome.css font.css jitsi_popover.css jquery-impromptu.css login_menu.css main.css modaldialog.css notice.css popover.css popup_menu.css settingsmenu.css toastr.css unsupported_browser.css videolayout_default.css welcome_page.css
 DEPLOY_DIR = libs
 BROWSERIFY_FLAGS = -d
 OUTPUT_DIR = .