Add -lstdc++fs to the link line of gcc

older versions of gcc (<8.1) need this, even if they accept -std=c++17

with this patch, the code can be run on fishtest again,
at least by the majority of workers (fishtest doesn't require c++17 to be available)

See e.g.
https://tests.stockfishchess.org/tests/view/5fcfbf801ac1691201888235

Bench: 3820648

src/Makefile

@@ -328,7 +328,7 @@ endif
 ifeq ($(COMP),gcc)
 	comp=gcc
 	CXX=g++
-	CXXFLAGS += -pedantic -Wextra -Wshadow
+	CXXFLAGS += -pedantic -Wextra -Wshadow -lstdc++fs
 
 	ifeq ($(arch),$(filter $(arch),armv7 armv8))
 		ifeq ($(OS),Android)