pzstd: fix linking for static builds

[jonringer]

When trying to build statically, you get the following error:

[ 69%] Linking CXX executable pzstd
x86_64-unknown-linux-musl-ld: cannot find -llibzstd_shared
collect2: error: ld returned 1 exit status

This is to correct the build when shared libraries aren't available. (e.g. BUILD_SHARED=OFF)

With changes:

[100%] Linking CXX executable pzstd
[100%] Built target pzstd

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[jonringer]

doing an else if may be more proper. But there's already an xor check on BUILD_SHARED and BUILD_STATIC

[jonringer]

failing test seems to be from a fuzz test getting stalled