Do not break backward compatibility

cxxopts.hpp is a single file and should not be in its own directory for
its namespace.

CMakeLists.txt

@@ -51,7 +51,7 @@ endif()
 add_library(cxxopts INTERFACE)
 target_sources(
   cxxopts INTERFACE
-  ${CMAKE_CURRENT_SOURCE_DIR}/include/cxxopts/cxxopts.hpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/include/cxxopts.hpp
   )
 target_include_directories(
   cxxopts INTERFACE

src/example.cpp

@@ -24,7 +24,7 @@ THE SOFTWARE.
 
 #include <iostream>
 
-#include "cxxopts/cxxopts.hpp"
+#include "cxxopts.hpp"
 
 int main(int argc, char* argv[])
 {

test/options.cpp

@@ -3,7 +3,7 @@
 
 #include <initializer_list>
 
-#include "cxxopts/cxxopts.hpp"
+#include "cxxopts.hpp"
 
 class Argv {
   public: