Fix eaassert.h include guard

[IncludeGuardian]

eaassert.h does not have a strict enough include guard to trigger the multiple inclusion optimization on most compilers.

The #if defined(EA_PRAGMA_ONCE_SUPPORTED) check occurs before any include of eacompilertraits.h (where EA_PRAGMA_ONCE_SUPPORTED is defined) so the preprocessor sees the #pragma once only if eaassert.h is included after another header that includes eacompilertraits.h.

The traditional include guard is also not strict enough to trigger the multiple-inclusion optimization as the only tokens that can appear outside of the #ifdef/#endif pair are whitespace, comments, and the null directive # (for certain compilers https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html). However, the previously mentioned EA_PRAGMA_ONCE_SUPPORTED check disables this optimization.

This commit moves the #pragma once code after the include directive where EA_PRAGMA_ONCE_SUPPORTED would be defined. It also tidies up the traditional include guard so that it satisfies the stricter rules mentioned above.

This was issues was found with IncludeGuardian 0.0.7.

[IncludeGuardian]

The full output from IncludeGuardian (https://includeguardian.io) with this particular issue highlighted can be found here https://gist.github.com/IncludeGuardian/a3716704237848e258471471b058879a#file-eastl-yaml-L160-L161

[IncludeGuardian]

@grojo-ea Would anyone be able to take a look at this?