util/macros: use -mno-ms-bitfields on MinGW instead of gcc_struct
gets rid of warning: 'gcc_struct' attribute ignored [-Wattributes] introduced by !23338 (merged)
According to the doc using -mno-ms-bitfields is equivalent to using gcc_struct https://gcc.gnu.org/onlinedocs/gcc/x86-Variable-Attributes.html
Edited by xantares