Add vmlinux.h submodule

Add a git submodule with the data from the vmlinux.h repository, as a
means to provide vmlinux.h headers for various architectures *FOR
TESTING PURPOSES*  from a more or less centrally-managed location.

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "blazesym"]
 	path = blazesym
 	url = https://github.com/libbpf/blazesym.git
+[submodule "vmlinux.h"]
+	path = vmlinux.h
+	url = https://github.com/libbpf/vmlinux.h.git