Treat narrowing conversion as warning, not error

Author: PaulStoffregen

Diff for: platform.txt

@@ -25,7 +25,7 @@ compiler.c.elf.flags={compiler.warning_flags} -Os -g -flto -fuse-linker-plugin -
 compiler.c.elf.cmd=avr-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp -flto -MMD
 compiler.cpp.cmd=avr-g++
-compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD -flto
+compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -Wno-error=narrowing -MMD -flto
 compiler.ar.cmd=avr-gcc-ar
 compiler.ar.flags=rcs
 compiler.objcopy.cmd=avr-objcopy