LibCrypt: Start installing headers again

This was accidentally broken in
ac401970475cd3ceb2c284a6072d91ca5d9de23e, after which we started
installing headers into a `LibCrypt` subdirectory instead.

`serenity_install_headers("")` is really the only thing that we need
from `serenity_libc`, so just replicate that manually.

Userland/Libraries/LibCrypt/CMakeLists.txt

@@ -11,4 +11,5 @@ set(SOURCES
 )
 
 serenity_lib(LibCrypt crypt)
+serenity_install_headers("")
 target_link_libraries(LibCrypt PRIVATE LibCryptSHA2)