build-sys: correct blkid.h includes

When using pkg-config to determine the include flags for blkid the
flags are returned as:

    $ pkg-config blkid --cflags
    -I/usr/include/blkid -I/usr/include/uuid

We use the <blkid/blkid.h> include which would be correct when using
the default compiler /usr/include header search path. However, when
cross-compiling the blkid.h will not be installed at /usr/include and
highly likely in a temporary system root. It is futher compounded if
the cross-compile packages are split up and the blkid package is not
available in the same sysroot as the compiler.

Regardless of the compilation setup, the correct include path should be
<blkid.h> if using the pkg-config returned CFLAGS.

src/basic/blkid-util.h

@@ -20,7 +20,7 @@
 ***/
 
 #ifdef HAVE_BLKID
-#include <blkid/blkid.h>
+#include <blkid.h>
 #endif
 
 #include "util.h"

src/boot/bootctl.c

@@ -19,7 +19,7 @@
 ***/
 
 #include <assert.h>
-#include <blkid/blkid.h>
+#include <blkid.h>
 #include <ctype.h>
 #include <dirent.h>
 #include <errno.h>

src/gpt-auto-generator/gpt-auto-generator.c

@@ -17,7 +17,7 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <blkid/blkid.h>
+#include <blkid.h>
 #include <stdlib.h>
 #include <sys/statfs.h>
 #include <unistd.h>

src/nspawn/nspawn.c

@@ -18,7 +18,7 @@
 ***/
 
 #ifdef HAVE_BLKID
-#include <blkid/blkid.h>
+#include <blkid.h>
 #endif
 #include <errno.h>
 #include <getopt.h>

src/udev/udev-builtin-blkid.c

@@ -18,7 +18,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include <blkid/blkid.h>
+#include <blkid.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <getopt.h>