PortAudio: Add missing -lsndio flag to fix build failure on Linux

Fixes a compilation error introduced in sysprog21#76, where the `make all` target fails
during the build of the PortAudio library on GNU/Linux systems.

The failure was caused by a missing `-lsndio` linker flag. Adding this flag
resolves the issue and allows the build to complete successfully.

Author: shengwen-tw

Makefile

@@ -81,7 +81,7 @@ ifeq ($(call has, VIRTIOSND), 1)
     LDFLAGS += $(PORTAUDIOLIB)
 
     ifeq ($(UNAME_S),Linux)
-        LDFLAGS += -lasound -lrt
+        LDFLAGS += -lasound -lrt -lsndio
         # Check PulseAudio installation
         ifeq (0, $(call check-pa))
             LDFLAGS += -lpulse