Makefile: fix string concatenation

Makefile

@@ -10,7 +10,7 @@ OS := $(shell uname)
 TAGS = ""
 BUILD_FLAGS = "-v"
 ifeq ($(OS),Linux)
-	BUILD_FLAGS += " -buildmode=pie"
+	BUILD_FLAGS += "-buildmode=pie"
 endif
 
 RELEASE_ROOT = "release"