concepts: Add header for common concepts

Initially, this is just a concept for the presence of an equals
operator.

Change-Id: I2838b589fa22f60489bba6a05ee56909c86c14b9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/meson.build b/include/meson.build
index 56f9c8e..a31fae5 100644
--- a/include/meson.build
+++ b/include/meson.build
@@ -2,6 +2,7 @@
 
 install_headers(
   'stdplus/cancel.hpp',
+  'stdplus/concepts.hpp',
   'stdplus/exception.hpp',
   'stdplus/flags.hpp',
   'stdplus/handle/copyable.hpp',