docs: Makefile: use the right path for DOCS_CSS

When make is used with O=<dir>, the location of the css file
won't be get right:

	$ make DOCS_THEME=nature DOCS_CSS=my_css.css    O=DOCS SPHINXDIRS=x86 -j9 htmldocs
	make[1]: Entering directory '/work/lnx/next/next-2021-1210/DOCS'
	...
	cp: cannot stat 'my_css.css': No such file or directory

Fix it in a way that both relative and absolute paths will be
handled.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/cea4ff1237ae9a99bc6509ab1bf9c70acd97e265.1639212812.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>