mk: fix dynamic linking option for board linux

-pthread is the one supposed to be used because it defines pre-defined
macros as -lpthread doesn't

mk/board_linux.mk

@@ -3,6 +3,6 @@ TOOLCHAIN = NATIVE
 include $(MK_DIR)/find_tools.mk
 
 # Linux build is just the same as SITL for now
-LIBS = -lm -lpthread -lrt
+LIBS = -lm -pthread -lrt
 include $(MK_DIR)/board_native.mk
 include $(MK_DIR)/upload_firmware.mk