[PATCH] scripts/kernel-doc: don't use uninitialized SRCTREE

Current kernel-doc (perl) script generates this warning:
Use of uninitialized value in concatenation (.) or string at scripts/kernel-doc line 1668.

So explicitly check for SRCTREE in the ENV before using it,
and then if it is set, append a '/' to the end of it, otherwise
the SRCTREE + filename can (will) be missing the intermediate '/'.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>