minor fix in TLS error message

include/grpc/support/port_platform.h

@@ -235,7 +235,7 @@
 #endif
 
 #if defined(GPR_MSVC_TLS) + defined(GPR_GCC_TLS) + defined(GPR_PTHREAD_TLS) + defined(GPR_CUSTOM_TLS) != 1
-#error Must define exactly one of GPR_MSVC_TLS, GPR_GCC_TLS, GPR_PTHREAD_TLS, defined(GPR_CUSTOM_TLS)
+#error Must define exactly one of GPR_MSVC_TLS, GPR_GCC_TLS, GPR_PTHREAD_TLS, GPR_CUSTOM_TLS
 #endif
 
 typedef int16_t gpr_int16;