ccache.conf: Add include_file_ctime to sloppiness

ccache.conf: Add include_file_ctime to sloppiness

When multiple recipes are built in parallel, Ccache sometimes refuses to
lookup some objects in cache, leading to undesired cache misses. The
root cause of this is an interaction between the way how bitbake
constructs a recipe sysroot and Ccache's `include_file_ctime` check.

Whenever bitbake creates a recipe's sysroot it hardlinks the files
provided by a recipes dependencies. Adding a hardlink to a file changes
it's ctime which in turn leads Ccache to believe that the file was
modified thus aborting the cache lookup.

To avoid this situation, add `include_file_ctime` to the list of checks
that should be ignored using the Ccache sloppiness configuration option
[1].

Example of a log entry that Ccache ignores a file:
/
|recipe-sysroot/usr/include/bits/pthread_stack_min.h had status change
|near or after invocation (ctime 1739822508.107677255, invocation time
|1739822507.970071107)
\

1 - https://ccache.dev/manual/4.10.2.html#config_sloppiness

Signed-off-by: Fabio Berton <fabio.berton@criticaltechworks.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>