[libc] Fix compile options for algorithm test

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.

Differential Revision: https://reviews.llvm.org/D128742

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

@@ -12,6 +12,7 @@ add_libc_unittest(
   COMPILE_OPTIONS
     ${LIBC_COMPILE_OPTIONS_NATIVE}
     -ffreestanding
+    -pthread
   DEPENDS
     libc.src.string.memory_utils.memory_utils
     libc.src.__support.CPP.array