libdpkg: Always include <stddef.h>

We need the definitions from the header, so conditionally including it
will only cause problems, we are also unconditionally including it in
other parts of the code, so it's safe to assume it's always available.

lib/dpkg/dpkg.h

@@ -28,9 +28,7 @@
 #include <sys/types.h>
 
 #include <setjmp.h>
-#ifdef HAVE_STDDEF_H
 #include <stddef.h>
-#endif
 #include <stdarg.h>
 #include <stdio.h>
 

lib/dpkg/path.h

@@ -21,9 +21,7 @@
 #ifndef LIBDPKG_PATH_H
 #define LIBDPKG_PATH_H
 
-#ifdef HAVE_STDDEF_H
 #include <stddef.h>
-#endif
 
 #include <dpkg/macros.h>