net: fjes: correct TRACE_INCLUDE_PATH

A comment in define_trace.h clearly states:

TRACE_INCLUDE_PATH if the path is something other than core kernel
vvvvvvvvvvvvvv
include/trace then this macro can define the path to use. Note, the path
is relative to define_trace.h, not the file including it. Full path names
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
for out of tree modules must be used.

fjes uses path relative to itself. Which (somehow) works most of
the time. Except when the kernel tree is "nested" in another
kernel tree, and ../drivers/net/fjes actually exists. In which
case build will use the header file from the wrong directory.

I've been trying to figure out why net NIPA builder is constantly
failing for the last 5 days, with:

include/trace/../../../drivers/net/fjes/fjes_trace.h:88:17: error: ‘__assign_str’ undeclared (first use in this function)
88 | __assign_str(err, err);
| ^~~~~~~~~~~~

when the line in the tree clearly has only one "err". NIPA does
indeed have "nested" trees, because it uses git work-trees and
the tree on the "outside" is not very up to date.

Link: https://lore.kernel.org/r/20240529023322.3467755-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>