Do not include libraries in goto-instrument.a

The library archive in goto-instrument included several other archives, which
caused spurious failures ("file format not recognized") when trying to link unit
tests.

Author: tautschnig

src/goto-instrument/Makefile

@@ -112,7 +112,7 @@ endif
 goto-instrument$(EXEEXT): $(OBJ)
 	$(LINKBIN)
 
-goto-instrument$(LIBEXT): $(OBJ)
+goto-instrument$(LIBEXT): $(SRC:.cpp=$(OBJEXT))
 	$(LINKLIB)
 
 .PHONY: goto-instrument-mac-signed