rebalancer manifest: enable position-independent code by default

Summary:
Add [cmake.defines] CMAKE_POSITION_INDEPENDENT_CODE=ON so that
rebalancer's library archives are PIC. This is required because
librebalancer.so is now built as a shared library; without PIC the
final link fails with R_X86_64_PC32 / 'recompile with -fPIC' against
its own intermediate object files.

Reviewed By: uhebertj

Differential Revision: D103767968

fbshipit-source-id: 05f290f6576fd843805b2dbeb8dd097a9f0de7e1

Author: r-barnes

build/fbcode_builder/manifests/rebalancer

@@ -14,6 +14,9 @@ branch = main
 [build]
 builder = cmake
 
+[cmake.defines]
+CMAKE_POSITION_INDEPENDENT_CODE=ON
+
 [dependencies]
 boost
 fbthrift