Add missing import required to use a conformance

A new compiler check reports a missing import for a conformance
implicitly imported:

```
Sources/_TestSupport/Error.swift:21:13: error: cannot use conformance of
'Float' to 'Real' here; 'RealModule' was not imported by this file
public func componentwiseError(_ tst: Complex<Float>, _ ref:
Complex<Double>) -> Double {
            ^
```

Address this issue in the project by adding the missing import.

Author: xymus

Sources/_TestSupport/Error.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import ComplexModule
+import RealModule
 
 public func relativeError(_ tst: Float, _ ref: Double) -> Double {
   let scale = max(ref.magnitude, Double(Float.leastNormalMagnitude))