Fixed warning.

src/bgfx.cpp

@@ -1518,7 +1518,7 @@ namespace bgfx
 
 	void UniformBuffer::writeUniform(UniformType::Enum _type, uint16_t _loc, const void* _value, uint16_t _num)
 	{
-		const uint32_t opcode = encodeOpcode(_type, _loc, _num, true);
+		const uint32_t opcode = encodeOpcode(bx::narrowCast<uint8_t>(_type), _loc, _num, true);
 		write(opcode);
 		write(_value, g_uniformTypeSize[_type]*_num);
 	}