Makefile: fix -ldflags -X invocation

Aerc uses the go linker to add some variables like main.Prefix / main.ShareDir
etc. however the go linker only considers the last -ldflags invocation it seems.

This commit fixes the makefile to use only a single -ldflags invocation, which
again sets the vars as expected.

Makefile

@@ -17,9 +17,9 @@ GOSRC+=go.mod go.sum
 
 aerc: $(GOSRC)
 	go build $(GOFLAGS) \
-		-ldflags "-X main.Prefix=$(PREFIX)" \
-		-ldflags "-X main.ShareDir=$(SHAREDIR)" \
-		-ldflags "-X main.Version=$(VERSION)" \
+		-ldflags "-X main.Prefix=$(PREFIX) \
+		-X main.ShareDir=$(SHAREDIR) \
+		-X main.Version=$(VERSION)" \
 		-o $@
 
 aerc.conf: config/aerc.conf.in