package/libcpprestsdk: zlib is optional, not mandatory

package/libcpprestsdk: zlib is optional, not mandatory

The CMakeLists.txt contains:

  if(CPPREST_EXCLUDE_COMPRESSION)
  if(NOT CPPREST_EXCLUDE_BROTLI)
  message(FATAL_ERROR "Use of Brotli requires compression to be enabled")
  endif()

libcpprestsdk requires openssl, so if the libopenssl provider is
chosen, it anyway pulls in zlib. But if libressl is used as a
provider, it doesn't pull in zlib, and building libcpprestsdk becomes
possible without zlib thanks to this patch.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>