thrift: SimpleJSONProtocol.tcc: avoid -Wfloat-conversion warnings
thrift: SimpleJSONProtocol.tcc: avoid -Wfloat-conversion warnings

Summary:
Avoid -Wfloat-conversion warnings.  This avoids errors like this when
building with -Werror -Wfloat-conversion:

  In file included from ./thrift/lib/cpp2/protocol/SimpleJSONProtocol.tcc:27:0,
  ./thrift/lib/cpp2/protocol/SimpleJSONProtocol.tcc: In member function 'uint32_t apache::thrift::SimpleJSONProtocolReader::readJSONVal(float&)':

Reviewed By: Gownta, yfeldblum

Differential Revision: D2704119

fb-gh-sync-id: e7b0a4c89495aebeb4ded79b7259e451042fec08

getyourguide/fbthrift,LinusU/fbthrift,Orvid/fbthrift,facebook/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,facebook/fbthrift,LinusU/fbthrift,Orvid/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,Orvid/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,Orvid/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,getyourguide/fbthrift,facebook/fbthrift,Orvid/fbthrift,LinusU/fbthrift,LinusU/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,Orvid/fbthrift,sinjar666/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,sinjar666/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,Orvid/fbthrift,Orvid/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,Orvid/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,LinusU/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,Orvid/fbthrift