configure: remove TMPDIR on exit

Commit e557d1ac ("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>

configure

@@ -5,7 +5,7 @@ INCLUDE=${1:-"$PWD/include"}
 
 # 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()
 {