linux: run depmod only if modules are enabled

linux: run depmod only if modules are enabled

If the kernel configuration CONFIG_MODULES disabled, there's no
/lib/modules/<version> directory, causing this:

depmod: ERROR: could not open directory [...]/target/lib/modules/4.19.8: No such file or directory
depmod: FATAL: could not search modules: No such file or directory
Makefile:745: recipe for target 'target-finalize' failed

Fixes:

https://gitlab.com/buildroot.org/buildroot/-/jobs/483712831
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712817
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712814
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712796
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712729
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712727
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712698
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712691
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712660
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712573
https://gitlab.com/buildroot.org/buildroot/-/jobs/483712574

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>