Fix spinel platform include on macos. (#1891)

diff --git a/src/ncp/Makefile.am b/src/ncp/Makefile.am
index e9aa0d9..1c335af 100644
--- a/src/ncp/Makefile.am
+++ b/src/ncp/Makefile.am
@@ -46,7 +46,7 @@
     -I$(top_srcdir)/src/core                        \
     -I$(top_srcdir)/third_party                     \
     -D_GNU_SOURCE                                   \
-    '-DSPINEL_PLATFORM_HEADER="spinel_platform.h"'  \
+    -DSPINEL_PLATFORM_HEADER=\"spinel_platform.h\"  \
     $(OPENTHREAD_TARGET_DEFINES)                    \
     $(NULL)