make: fix PATH escaping

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

Makefile

 # General tools
 
-SHELL=PATH=$(PATH) /bin/sh
+SHELL=PATH='$(PATH)' /bin/sh
 
 PROTOC = protoc --gogo_out=. --proto_path=.:/usr/local/opt/protobuf/include:$(dir $@) $<