Add -Wsign-compare to the project's CFLAGS

[jgalar]

The lttng-tools code base has many erroneous comparisons between
signed and unsigned values.

While some uses are benign, like using a signed integer to loop
between 0 and sizeof(struct some_structure), there are many
cases where the "signed integer" result of a syscall
(e.g. sendmsg, recvmsg, read, write) is compared to an expected
structure size using the sizeof operator, which returns a result of type
size_t.

The following pattern:

ret = sendmsg(...);
if (ret < sizeof(struct some_structure)) {
	/* Handle error. */
}

is common and unsafe if the intention is to check for an error
(-1) and that the operation was completed (ret == sizeof(...)).

Follow-up patches address the various instances of the problem.

The Clean-up: prefix is used when the comparison was safe
and the change is mainly done to silence the warning.

The Fix: prefix is used when the commit fixes an unsafe
comparison and should be backported to previous versions.