file.c: move readahead parser to a separate file

* readahead.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* file.c (sys_readahead): Move to readahead.c.
diff --git a/Makefile.am b/Makefile.am
index 97266f0..0c03252 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -55,6 +55,7 @@
 	process.c	\
 	ptp.c		\
 	quota.c		\
+	readahead.c	\
 	readlink.c	\
 	reboot.c	\
 	renameat.c	\