bpf: Include linux/types.h for u32

Inclusion of the header linux/btf_ids.h relies on indirect inclusion of
the header linux/types.h. Including it directly on the top level helps
to avoid potential problems if linux/types.h hasn't been included
before.

The main motivation to introduce this it is to avoid similar problems that
have shown up in the bpftool where GNU libc indirectly pulls
linux/types.h causing compile error of the form:

   error: unknown type name 'u32'
                             u32 cnt;
                             ^~~

The bpftool compile error was fixed in
62248b22 ("tools/resolve_btfids: fix build with musl libc").
Signed-off-by: Dmitrii Bundin <dmitrii.bundin.a@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240420042457.3198883-1-dmitrii.bundin.a@gmail.com

include/linux/btf_ids.h

@@ -3,6 +3,8 @@
 #ifndef _LINUX_BTF_IDS_H
 #define _LINUX_BTF_IDS_H
 
+#include <linux/types.h> /* for u32 */
+
 struct btf_id_set {
 	u32 cnt;
 	u32 ids[];