fix include loop on MinGW due to libintl.h pulling stdio.h

fix include loop on MinGW due to libintl.h pulling stdio.h

In file included from ./include/grub/dl.h:23:0,
                 from grub-core/lib/libgcrypt-grub/cipher/rfc2268.c:3:
./include/grub/list.h:34:18: warning: conflicting types for 'grub_list_push' [en
abled by default]
 void EXPORT_FUNC(grub_list_push) (grub_list_t *head, grub_list_t item);
                  ^
./include/grub/symbol.h:68:25: note: in definition of macro 'EXPORT_FUNC'
 # define EXPORT_FUNC(x) x
                         ^
In file included from ./include/grub/fs.h:30:0,
                 from ./include/grub/file.h:25,
                 from ./grub-core/lib/posix_wrap/stdio.h:23,
                 from c:\mingw\include\libintl.h:314,
                 from ./include/grub/i18n.h:33,
                 from ./include/grub/misc.h:27,
                 from ./include/grub/list.h:25,
                 from ./include/grub/dl.h:28,
                 from grub-core/lib/libgcrypt-grub/cipher/rfc2268.c:3:
./include/grub/partition.h:106:3: note: previous implicit declaration of 'grub_l
ist_push' was here
   grub_list_push (GRUB_AS_LIST_P (&grub_partition_map_list),
   ^
list.h needs just ATTRIBUTE_ERROR from misc.h; split compiler features
into separate file grub/compiler.h and include it instead.