Fix extension of files referenced by header

[awesomebytes]

I... don't really know how this was working for anyone?

Or maybe it's a weird behaviour of my system and .h and .hpp should be detected anyways?

Oh well, this fixed it for me. It corrects the error (for google searches):

Errors     << image_transport_py:make /home/sam/workspaces/roscpp_py_ws/logs/image_transport_py/build.make.061.log
In file included from /home/sam/workspaces/roscpp_py_ws/src/image_transport_py/src/image_transport_py.cpp:7:0:
/home/sam/workspaces/roscpp_py_ws/src/pyrosmsg/include/pyrosmsg/pyrosmsg.h:4:10: fatal error: pyrosmsg/converters.h: No such file or directory
 #include "pyrosmsg/converters.h"

It took me embarrassingly long to figure it out.

[awesomebytes]

I also remembered another error that was quickly fixed:

Errors     << pyrosmsg:check /home/sam/workspaces/roscpp_py_ws/logs/pyrosmsg/build.check.071.log
CMake Error at /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:83 (find_package):
  Could not find a package configuration file provided by "pybind11" with any
  of the following names:

    pybind11Config.cmake
    pybind11-config.cmake

  Add the installation prefix of "pybind11" to CMAKE_PREFIX_PATH or set
  "pybind11_DIR" to a directory containing one of the above files.  If
  "pybind11" provides a separate development package or SDK, be sure it has
  been installed.
Call Stack (most recent call first):
  CMakeLists.txt:6 (find_package)

(Added to this PR)