Add libextension_memory_allocator.a to executorch_llm Apple framework (#19200)

### Summary
PR #15730 added a dependency from extension/llm/runner on
extension::CPUCachingAllocator (defined in
libextension_memory_allocator.a) but did not update
scripts/build_apple_frameworks.sh, so the executorch_llm iOS framework
was missing the symbol. This caused the Benchmark Tests target to fail
with `Undefined symbols: CPUCachingAllocator(unsigned int)`, breaking
the Apple/build-benchmark-app job on trunk.

Authored with the assistance of Claude (Anthropic).

### Test plan
CI

Author: rascani

scripts/build_apple_frameworks.sh

@@ -80,6 +80,7 @@ libabsl_tracing_internal.a,\
 libabsl_utf8_for_code_point.a,\
 libextension_llm_apple.a,\
 libextension_llm_runner.a,\
+libextension_memory_allocator.a,\
 libpcre2-8.a,\
 libre2.a,\
 libregex_lookahead.a,\