Add missing c file to Package.swift

[kchro3]

The latest master can't import llama in Xcode properly. It fails with linker errors and can't resolve functions:

Undefined symbol: _ggml_allocr_alloc

Undefined symbol: _ggml_allocr_alloc_graph

Undefined symbol: _ggml_allocr_free

Undefined symbol: _ggml_allocr_is_measure

Undefined symbol: _ggml_allocr_new

Undefined symbol: _ggml_allocr_new_measure

Undefined symbol: _ggml_allocr_reset

This can be solved by adding the ggml-alloc.c to the Package.swift