exporters/jaeger: include unistd.h for close()

`close()` is declared in `<unistd.h>`. Without this header, the
build fails with both gcc and clang.

The Jaeger exporter has been removed upstream, so this fix is
not submitted upstream.

clang:
> exporters/jaeger/src/TUDPTransport.cc:71:7: error: no type named 'close' in the global namespace
>    71 |     ::THRIFT_CLOSESOCKET(socket_);
>       |     ~~^
> /usr/include/thrift/transport/PlatformSocket.h:102:30: note: expanded from macro 'THRIFT_CLOSESOCKET'
>   102 | #  define THRIFT_CLOSESOCKET close
>       |                              ^

gcc:
> In file included from exporters/jaeger/src/TUDPTransport.h:16,
>                  from exporters/jaeger/src/TUDPTransport.cc:6:
> exporters/jaeger/src/TUDPTransport.cc: In member function ‘virtual void opentelemetry::v1::exporter::jaeger::TUDPTransport::close()’:
> exporters/jaeger/src/TUDPTransport.cc:71:7: error: ‘::close’ has not been declared; did you mean ‘pclose’?
>    71 |     ::THRIFT_CLOSESOCKET(socket_);
>       |       ^~~~~~~~~~~~~~~~~~
--- a/exporters/jaeger/src/TUDPTransport.h
+++ b/exporters/jaeger/src/TUDPTransport.h
@@ -10,6 +10,7 @@
 #  include <string.h>
 #  include <sys/socket.h>
 #  include <sys/types.h>
+#  include <unistd.h>
 #endif
 
 #include <opentelemetry/version.h>