README.md: Add QNX to the formerly supported OS list. [skip ci]

infrastation · infrastation · commit 4822cd1f88f7 · 2024-04-24T22:56:35.000+01:00
There are various mentions of QNX implementing BPF and running tcpdump,
but finding an actual running OS and verifying it seems complicated.
diff --git a/README.md b/README.md
@@ -48,6 +48,7 @@ In the past tcpdump certainly or likely worked on the following platforms:
 * DOS
 * IRIX
 * LynxOS
+* QNX
 * SINIX
 * SunOS
 * Ultrix
