src/libais/Makefile-custom: Bring back -Wno-sign-compare

```
g++ -std=c++20 -g -O2 -Wall -Wextra -Werror   -c -o ais4_11.o ais4_11.cpp
In file included from /usr/include/c++/13/cassert:44,
                 from ais.h:10,
                 from ais_bitset.cpp:9:
ais_bitset.cpp: In member function ‘const libais::AisBitset& libais::AisBitset::SeekTo(size_t) const’:
ais_bitset.cpp:64:14: error: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘const int’ [-Werror=sign-compare]
   64 |   assert(pos < num_bits);
      |          ~~~~^~~~~~~~~~
```

Author: schwehr

src/libais/Makefile-custom

@@ -11,6 +11,7 @@ CXXFLAGS += -std=c++20
 
 WARN := -Wall
 WARN += -Wextra
+WARN += -Wno-sign-compare
 WARN += -Werror
 
 # Disabled warnings