Fix doc target missing declarations errors

Building the docs (e.g. with `ninja doc`) failed with GCC (e.g. GCC 7.3
under Fedora 27) with some missing declaration errors.

Example:

    ../Rx/v2/examples/doxygen/main.cpp:7:13: error: no previous
    declaration for ‘std::__cxx11::string get_pid()’
    [-Werror=missing-declarations]

Thus, this change adds a shared declaration for `get_pid()` that is
included by all users/the file where it's defined such that
accidental deviation in the signature are immediately noticed (as
compile error).

Similarly, the visibility of the example `less()` is changed to static
as it's only locally used.

With those changes the doc target succeeds again.

1 files changed, 1 insertions, 1 deletions

diff --git a/Rx/v2/examples/doxygen/observe_on.cpp b/Rx/v2/examples/doxygen/observe_on.cpp
index 927c339..9046999 100644
--- a/Rx/v2/examples/doxygen/observe_on.cpp
+++ b/Rx/v2/examples/doxygen/observe_on.cpp
@@ -3,7 +3,7 @@
 #include "rxcpp/rx-test.hpp"
 #include "catch.hpp"
 
-std::string get_pid();
+#include "main.hpp"
 
 SCENARIO("observe_on sample"){
     printf("//! [observe_on sample]\n");