Add xrpl.libpb as an exported lib

[godexsoft]

High Level Overview of Change

Tiny change to export libpb so that consumers of libxrpl can link against it again.

Context of Change

Clio could not link against libxrpl produced from the current code.
The reason seems to be that libpb was not exported and consequently did not make it into the linked libraries despite being publicly linked thru thexrpl::libxrpl target in cmake.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

None other than being able to link against libxrpl as a consumer.

• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.1%. Comparing base (f2d37da) to head (9bcfc85).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5022   +/-   ##
=======================================
  Coverage     71.1%   71.1%           
=======================================
  Files          796     796           
  Lines        66997   66997           
  Branches     10980   10978    -2     
=======================================
+ Hits         47634   47636    +2     
+ Misses       19363   19361    -2     

see 2 files with indirect coverage changes