[using.linkage] Fix reference to section of C standard in footnote

[sdutoit]

From LWG Issue 2281:

Edit footnote 182, 17.6.2.3 [using.linkage] as indicated:

The only reliable way to declare an object or function signature from the Standard C library is by including the header that declares it, notwithstanding the latitude granted in 7.1.77.1.4 of the C Standard.

[AlisdairM]

The current draft has the suggested 7.1.4 cross-reference, so this issue can be safely closed.