package/librtlsdr: bump to last commit

package/librtlsdr: bump to last commit

Last official release is 2 years ago.

Current master branch provides fixes and improvements:
- fix crash with usbfs mmap with usb controler has no DMA support and kernel < 5.6 [1].
- better SDR dongle configuration

This patch:
- bump librtlsdr to d794155ba65796a76cd0a436f9709f4601509320 (2020/03/18)
- adapt 0001-disable_shared_library_target_in_build.patch
  (http://lists.osmocom.org/pipermail/osmocom-sdr/2020-May/002075.html)
- add option to enable/disable zerocopy buffer on non arm platform

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/usb/core?h=v5.8-rc3&id=a0e710a7def471b8eb779ff551fc27701da49599

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>