Fix cmake abosule paths in build interface

[AgarwalSaurav]

GLOB creates absolute paths requiring the BUILD_INTERFACE generator to be placed inside quotes.

Currently, here is what the autogenerated TorchSparseTargets.cmake looks like:

set_target_properties(TorchSparse::TorchSparse PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "/home/user/opt/pytorch_sparse/"
)

This causes an error when the TorchSparse::TorchSparse is added to other projects because of the absolute path.

Adding quotes to the generator in CMakeLists.txt fixes this issue.

target_include_directories(${PROJECT_NAME} INTERFACE
  "$<BUILD_INTERFACE:${HEADERS}>"
  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)