add support for linux 4.19 kernel headers

both 4.19.90 from official kernel tarball and 4.19.88 from the
sabotage-linux headers-only package are added. the latter should be
preferred unless you have a reason not to, as it's much smaller and
has some patches that improve interaction with musl.

support for all earlier kernel header versions will be dropped soon,
since everything prior to 4.16 has incompatibilities with 32-bit archs
moving to time64.

hashes/linux-4.19.90.tar.xz.sha1

@@ -0,0 +1 @@
+2663224361a984c8a62b9cf9fc8c82c0a75f9f3f  linux-4.19.90.tar.xz

hashes/linux-headers-4.19.88.tar.xz.sha1

@@ -0,0 +1 @@
+de12b9c8ae2de9e85056a36be9f0fcc0a1e4abe9  linux-headers-4.19.88.tar.xz