fix cmake

Author: vinay-deshmukh

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -37,6 +37,7 @@ function(_get_common_test_compile_options output_var c_test flags)
       # list(APPEND compile_options "-Werror")
     endif()
     list(APPEND compile_options "-Wconversion")
+    list(APPEND compile_options "-Wsign-conversion")
     list(APPEND compile_options "-Wimplicit-fallthrough")
     list(APPEND compile_options "-Wwrite-strings")
     # Silence this warning because _Complex is a part of C99.