Revert " Increment major version"

This reverts commit 6d3d094.

Author: pszymich

IGC/CMakeLists.txt

@@ -32,7 +32,7 @@ endif()
 
 # Increment IGC_API_MAJOR_VERSION if there is ABI Change.
 if(NOT DEFINED IGC_API_MAJOR_VERSION)
-  set(IGC_API_MAJOR_VERSION 2)
+  set(IGC_API_MAJOR_VERSION 1)
 endif()
 if(NOT DEFINED IGC_API_MINOR_VERSION)
   set(IGC_API_MINOR_VERSION 0)