makefile: make sure we apppend to GOBULDFLAGS if user wants to add th…

…eir own.

Makefile

@@ -1,7 +1,7 @@
 
 GO=go
 GOLDFLAGS=-v -s -w -X main.VersionString=$(PROGRAM_VERSION)
-GOBUILDFLAGS=-v -p $(shell nproc) -ldflags="$(GOLDFLAGS)"
+GOBUILDFLAGS+=-v -p $(shell nproc) -ldflags="$(GOLDFLAGS)"
 PROGRAM=thalos-server
 PROGRAM_VERSION ?= 1.1.5
 PREFIX=/usr/local