[Shims] Fix nullability for musl.

[al45tair]

Apparently I got the merge for this slightly wrong.

rdar://128388845

[al45tair]

@swift-ci Please test

[al45tair]

Explanation: When merging my Musl changes, I hit a conflict with an Android change and apparently got the merge every so slightly wrong, the result being that we see the wrong declaration of memcmp() when building the static Linux SDK, which causes a build failure because the signature doesn't match (so instead memcmp() with the wrong signature ends up floating about in random modules). This only affects building the static Linux SDK; it doesn't affect anything else.
Original PR: #73747
Risk: Basically zero. This only affects building (but not using) the static Linux SDK.
Reviewed by: @mikeash
Resolves: rdar://128388845
Tests: Without this fix, building the Linux static SDK fails. With it, it succeeds.