Add codeowners of /src/solvers/makefile

So that those who can approve changes to the smt2 solver support can
also approve adding / removing / renaming `.cpp` files in those
subdirectories. The list of codeowners is based on the global code
owners plus the smt2 code owners. This is as suggested by Michael
Tautschnig here -
#6415 (comment)

Author: thomasspriggs

CODEOWNERS

@@ -56,6 +56,7 @@
 /src/cpp/ @kroening @tautschnig @peterschrammel
 /src/solvers/smt2 @kroening @martin-cs @peterschrammel @thomasspriggs @NlightNFotis @TGWDB
 /src/solvers/smt2_incremental @peterschrammel @thomasspriggs @NlightNFotis @TGWDB @chrisr-diffblue
+/src/solvers/makefile @kroening @tautschnig @peterschrammel @chrisr-diffblue @thomasspriggs @NlightNFotis @TGWDB
 /src/statement-list/ @kroening @tautschnig @peterschrammel @pkesseli
 
 /cmake/        @diffblue/diffblue-opensource