Use exec_file_find to prepend gdb_sysroot in follow_exec

Use exec_file_find to prepend gdb_sysroot in follow_exec

This commit updates follow_exec to use exec_file_find to prefix
the new executable's filename with gdb_sysroot rather than doing
it longhand.

gdb/ChangeLog:

	* infrun.c (solist.h): New include.
	(follow_exec): Use exec_file_find to prefix execd_pathname
	with gdb_sysroot.