buffer.h missing on mingw

While building to mingw:

```
make PREFIX=x86_64-w64-mingw32- --file=config-mingw.mk
x86_64-w64-mingw32-gcc --std=gnu89 -I./include -D_WIN32_WINNT=0x0600 -Isrc -g -c src/pattern.c -o src/pattern.o
In file included from src/pattern.c:2:
./include/fnmatch.h:37:10: fatal error: buffer.h: No such file or directory
#include <buffer.h>

```