libqrexec SONAME bump

The previous two changes were ABI breaks.

libqrexec/Makefile

@@ -9,7 +9,7 @@ override QUBES_CFLAGS := -I. -I../libqrexec -g -O2 -Wall -Wextra -Werror \
 
 override LDFLAGS += -pie -Wl,-z,relro,-z,now -shared
 
-override SO_VER=3
+override SO_VER=4
 override VCHANLIBS := $(shell pkg-config --libs vchan)
 LIBDIR ?= /usr/lib
 INCLUDEDIR ?= /usr/include