configure: remove TMPDIR on exit

Commit e557d1a ("Don't put configure files in /tmp") introduced a typo
that prevented automated cleanup of the temporary directory created for
feature testing. Fix this typo.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
diff --git a/configure b/configure
index db7cd6a..2c2d1c4 100755
--- a/configure
+++ b/configure
@@ -5,7 +5,7 @@
 
 # Make a temp directory in build tree.
 TMPDIR=$(mktemp -d config.XXXXXX)
-trap 'status=$?; rm -rf $TMPDIRa; exit $status' EXIT HUP INT QUIT TERM
+trap 'status=$?; rm -rf $TMPDIR; exit $status' EXIT HUP INT QUIT TERM
 
 check_atm()
 {