musl: meson: make musl not define wchar_t in their header

musl: meson: make musl not define wchar_t in their header

Otherwise, musl defines wchar_t as int, which conflicts with the
assumption by sd-boot, i.e. wchar_t is 2 bytes.

Fixes the following build error:
```
In file included from ../src/boot/efi-log.h:4,
                 from ../src/boot/linux_x86.c:13:
../src/boot/efi.h:19:24: error: conflicting types for 'wchar_t'; have 'short unsigned int'
   19 | typedef __WCHAR_TYPE__ wchar_t;
      |                        ^~~~~~~
In file included from /usr/include/stddef.h:19,
                 from ../src/boot/efi.h:9:
/usr/include/bits/alltypes.h:10:13: note: previous declaration of 'wchar_t' with type 'wchar_t' {aka 'int'}
   10 | typedef int wchar_t;
      |             ^~~~~~~
```