externalsrc.bbclass: Remove a trailing slash from ${B}

The trailing slash in ${B} caused -fdebug-prefix-map=${B}=... to not
match as intended, resulting in ${TMPDIR} ending up in files in
${PN}-dbg when externalsrc was in use, which in turn triggered buildpath
QA warnings.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>