Reorder include paths to ensure that runqslower sources are picking up
vmlinux.h, generated by runqslower's own Makefile. When runqslower is built
from selftests/bpf, due to current -I$(BPF_INCLUDE) -I$(OUTPUT) ordering, it
might pick up not-yet-complete vmlinux.h, generated by selftests Makefile,
which could lead to compilation errors like [0]. So ensure that -I$(OUTPUT)
goes first and rely on runqslower's Makefile own dependency chain to ensure
vmlinux.h is properly completed before source code relying on it is compiled.

  [0] https://travis-ci.org/github/libbpf/libbpf/jobs/677905925Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200422012407.176303-1-andriin@fb.com