Add missing header to fix build issues on gcc 13

[Dami-18]

Fixes #27

Added #include "ISequence.h" to PhasedBackoffWaitStrategy.cpp, bringing in the full class definition where .value() is actually called.

Testing

• Environment: Linux Mint (Ubuntu-based), GCC 13.3.0, Boost via system package
• Build command:

cmake .. -DCMAKE_BUILD_TYPE=Release -DDISRUPTOR_BUILD_TESTS=true
make -j$(nproc)

• Confirms clean build with no errors after the fix
• Existing test suite passes