qrexec: Disable all warnings being treated as errors

gcc -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -I. -g -Wall -Wextra -Werror -pie -fPIC `pkg-config --cflags vchan-xen` -D_FORTIFY_SOURCE=2  -c -o qrexec-agent-data.o qrexec-agent-data.c
qrexec-agent-data.c: In function 'handle_remote_data':
qrexec-agent-data.c:217:17: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
                 status = *(unsigned int *)buf;
                 ^
cc1: all warnings being treated as errors
<builtin>: recipe for target 'qrexec-agent-data.o' failed

--- a/qrexec/Makefile
+++ b/qrexec/Makefile
@@ -1,5 +1,5 @@
 CC=gcc
-CFLAGS+=-I. -g -Wall -Wextra -Werror -pie -fPIC `pkg-config --cflags vchan-$(BACKEND_VMM)`
+CFLAGS+=-I. -g -Wall -Wextra -pie -fPIC `pkg-config --cflags vchan-$(BACKEND_VMM)`
 LIBS=`pkg-config --libs vchan-$(BACKEND_VMM)` -lqrexec-utils
 
 all: qrexec-agent qrexec-client-vm