build: Create ell directory for ell/ell.h target

Both ell/shared and ell/internal targets first create the ell/
directory within IWD. This apparently was just luck that one of
these always finished first in parallel builds. On my system at
least when building using dpkg-buildpackage IWD fails to build
due to the ell/ directory missing. From the logs it appears that
both the shared/internal targets were started but didn't complete
(or at least create the directory) before the ell/ell.h target:

make[1]: Entering directory '/home/jprestwood/tmp/iwd'
/usr/bin/mkdir -p ell
/usr/bin/mkdir -p ell
echo -n > ell/ell.h
/usr/bin/mkdir -p src
/bin/bash: line 1: ell/ell.h: No such file or directory
make[1]: *** [Makefile:4028: ell/ell.h] Error 1

Creating the ell/ directory within the ell/ell.h target solve
the issue. For reference this is the configure command dpkg
is using:

./configure --build=x86_64-linux-gnu \
	--prefix=/usr \
	--includedir=/usr/include \
	--mandir=/usr/share/man \
	--infodir=/usr/share/info \
	--sysconfdir=/etc \
	--localstatedir=/var \
	--disable-option-checking \
	--disable-silent-rules \
	--libdir=/usr/lib/x86_64-linux-gnu \
	--runstatedir=/run \
	--disable-maintainer-mode \
	--disable-dependency-tracking \
	--enable-tools \
	--enable-dbus-policy

Makefile.am

@@ -711,6 +711,7 @@ ell/internal: Makefile
 	done > $@
 
 ell/ell.h: Makefile
+	$(AM_V_at)$(MKDIR_P) ell
 	$(AM_V_at)echo -n > $@
 	$(AM_V_GEN)for f in $(ell_headers) ; do \
 		echo "#include <$$f>" >> $@ ; \