Fix headerscheck and cpluspluscheck's exit codes.

For the benefit of CI, which started running these header check scripts
in its CompilerWarnings task in commit 81b9f23c9c, they should report
failure if any individual header failed to compile.

Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGKtDwPo9wzKgbStDwfOhEpywMc6PQofio8fAHR7yUjgxw%40mail.gmail.com

src/tools/pginclude/cpluspluscheck

@@ -52,6 +52,8 @@ tmp=`mktemp -d /tmp/$me.XXXXXX`
 
 trap 'rm -rf $tmp' 0 1 2 3 15
 
+exit_status=0
+
 # Scan all of src/ and contrib/ for header files.
 for f in `cd "$srcdir" && find src contrib -name '*.h' -print`
 do
@@ -167,9 +169,13 @@ do
 	esac
 
 	# Run the test.
-	${CXX:-g++} -I $builddir -I $srcdir \
+	if ! ${CXX:-g++} -I $builddir -I $srcdir \
 		-I $builddir/src/include -I $srcdir/src/include \
 		-I $builddir/src/interfaces/libpq -I $srcdir/src/interfaces/libpq \
 		$EXTRAINCLUDES $EXTRAFLAGS $CXXFLAGS -c $tmp/test.cpp
-
+	then
+		exit_status=1
+	fi
 done
+
+exit $exit_status