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.

(From OE-Core rev: e29509d855374995034d89b381e6ad9ffed90c23)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 9b5031ed5a0d102905fa75acc418246c23df6eef)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -60,7 +60,7 @@ python () {
         if externalsrcbuild:
             d.setVar('B', externalsrcbuild)
         else:
-            d.setVar('B', '${WORKDIR}/${BPN}-${PV}/')
+            d.setVar('B', '${WORKDIR}/${BPN}-${PV}')
 
         local_srcuri = []
         fetch = bb.fetch2.Fetch((d.getVar('SRC_URI') or '').split(), d)