kernel-api.rst: make it handle lib/crc32.c

This file has only "internal" functions:
	./lib/crc32.c:1: warning: no structured comments found
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/kernel-api.rst

@@ -80,7 +80,6 @@ CRC Functions
    :export:
 
 .. kernel-doc:: lib/crc32.c
-   :export:
 
 .. kernel-doc:: lib/crc-ccitt.c
    :export: