[libc] Fix compile options for algorithm test

[michaelrj]

Summary

This patch fixes the problem the bots were having with the algorithm
test not including pthreads correctly. They will likely need a manual
forced clean build for this to take effect.

libc/test/src/string/memory_utils/CMakeLists.txt

 add_libc_unittest(
   utils_test
   SUITE
     libc_string_unittests
   SRCS
     address_test.cpp
     algorithm_test.cpp
     backend_test.cpp
     elements_test.cpp
     memory_access_test.cpp
     utils_test.cpp
   COMPILE_OPTIONS
     ${LIBC_COMPILE_OPTIONS_NATIVE}
     -ffreestanding
+    -pthread
   DEPENDS
     libc.src.string.memory_utils.memory_utils
     libc.src.__support.CPP.array
     libc.src.__support.CPP.array_ref
 )