mingw: move poll out of sys-folder

Both XSI and upstream Gnulib versions expects to find poll.h at
the root of some include path, not inside the sys-folder.

This helps us when upgrading Gnulib.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1088,6 +1088,7 @@ ifeq ($(uname_S),Windows)
 	NO_PREAD = YesPlease
 	NEEDS_CRYPTO_WITH_SSL = YesPlease
 	NO_LIBGEN_H = YesPlease
+	NO_SYS_POLL_H = YesPlease
 	NO_SYMLINK_HEAD = YesPlease
 	NO_IPV6 = YesPlease
 	NO_SETENV = YesPlease
@@ -1126,7 +1127,7 @@ ifeq ($(uname_S),Windows)
 	BASIC_CFLAGS = -nologo -I. -I../zlib -Icompat/vcbuild -Icompat/vcbuild/include -DWIN32 -D_CONSOLE -DHAVE_STRING_H -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE
 	COMPAT_OBJS = compat/msvc.o compat/winansi.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
-		compat/win32/sys/poll.o compat/win32/dirent.o
+		compat/win32/poll.o compat/win32/dirent.o
 	COMPAT_CFLAGS = -D__USE_MINGW_ACCESS -DNOGDI -DHAVE_STRING_H -DHAVE_ALLOCA_H -Icompat -Icompat/regex -Icompat/win32 -DSTRIP_EXTENSION=\".exe\"
 	BASIC_LDFLAGS = -IGNORE:4217 -IGNORE:4049 -NOLOGO -SUBSYSTEM:CONSOLE -NODEFAULTLIB:MSVCRT.lib
 	EXTLIBS = user32.lib advapi32.lib shell32.lib wininet.lib ws2_32.lib
@@ -1181,6 +1182,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_PREAD = YesPlease
 	NEEDS_CRYPTO_WITH_SSL = YesPlease
 	NO_LIBGEN_H = YesPlease
+	NO_SYS_POLL_H = YesPlease
 	NO_SYMLINK_HEAD = YesPlease
 	NO_SETENV = YesPlease
 	NO_UNSETENV = YesPlease
@@ -1214,7 +1216,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
 	COMPAT_OBJS += compat/mingw.o compat/winansi.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
-		compat/win32/sys/poll.o compat/win32/dirent.o
+		compat/win32/poll.o compat/win32/dirent.o
 	EXTLIBS += -lws2_32
 	PTHREAD_LIBS =
 	X = .exe