add wrong warning to suppression file

support-files/compiler_warnings.supp:
  we want that.

support-files/compiler_warnings.supp

@@ -79,6 +79,7 @@ mi_packrec.c : .*result of 32-bit shift implicitly converted to 64 bits.* : 567
 # Wrong compiler warnings
 #
 .* : .*no matching operator delete found; memory will not be freed if initialization throws an exception.*
+ctype-simple.c : .*unary minus operator applied to unsigned type, result still unsigned.*
 
 #
 # Viossl warnings - fixed in 5.1, disabled in 5.0. Too large to be changed