blktap2: use sys/eventfd.h if it is available

arm64 only has the eventfd2 system call and using the libc wrapper when
available hides this from us. eventfd() has been in libc since glibc 2.8.

This code is already Linux specific.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/config.h.in

@@ -27,6 +27,9 @@
 /* Define to 1 if you have the <string.h> header file. */
 #undef HAVE_STRING_H
 
+/* Define to 1 if you have the <sys/eventfd.h> header file. */
+#undef HAVE_SYS_EVENTFD_H
+
 /* Define to 1 if you have the <sys/stat.h> header file. */
 #undef HAVE_SYS_STAT_H