Make global-context-less-secure actually enable the global context (fixup for #407)

[elichai]

In #407 we restored the global-context-less-secure feature, but it didn't actually do anything because #385 changed all the cfg checks on the whole module to depend on global-context, so we need to enable global-context in order to make that module compile.

so before all this, users could enable just global-context-less-secure without enabling the global-context, and after this PR it will behave the same.

(this will not enable the randomization because of:

rust-secp256k1/src/context.rs

Line 51 in 1cf2429

#[cfg(all(feature = "rand-std", not(feature = "global-context-less-secure")))]

)

[apoelstra]

ACK c1bb316

Good catch.