formula: make operator bool explicit

* spot/tl/formula.hh (formula::operator bool): Make it explicit.  When
compiling in C++20 mode with g++ 10.1, this bool operator was used
instead of the comparison operators while looking up a
std::pair<formula,formula> in a hash map, causing many test suite
failures.  This problem does not occur with clang++ 10.0, so it
might just be a bug in g++ 10.1.  But having explicit operator bool
is good practice anyway.

spot/tl/formula.hh

@@ -842,7 +842,7 @@ namespace spot
       return ptr_ != nullptr;
     }
 
-    operator bool() const noexcept
+    explicit operator bool() const noexcept
     {
       return ptr_ != nullptr;
     }