kernel-doc: Strip #ifdef/#endif in enums

Some enumerations in the kernel headers use #ifdef to reduce their size
based on the the configuration. These lines have to be stripped to avoid
parsing problems.

For example a simple input like

    /**
     * enum flags - test flags
     * @flag1: first flag
     * @flag2: second flag
     */
    enum flags {
    	flag1 = BIT(0),
    #ifdef SECOND_FLAG
    	flag2 = BIT(1),
    #endif
    };

resulted in parsing warnings like

    warning: Enum value '#ifdef SECOND_FLAG;flag2 = BIT(1)' not described in enum 'flags'
    warning: Enum value '#endif;' not described in enum 'flags'

Signed-off-by: Conchúr Navid <conchur@web.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>