singles: make MagnumMath.hpp depend on CorradePair.h.

This makes the library go from slightly over 10k preprocessed lines to
9.5, yay!

doc/singles.dox

@@ -46,7 +46,7 @@ sources using the builtin @ref acme "single-header generator tool".
 
 Class or library            | Single-header version
 --------------------------- | ----------------------------
-@ref Math, \n @ref EigenIntegration, \n @ref GlmIntegration | [MagnumMath.hpp](https://github.com/mosra/magnum-singles/tree/master/MagnumMath.hpp)
+@ref Math, \n @ref EigenIntegration, \n @ref GlmIntegration | [MagnumMath.hpp](https://github.com/mosra/magnum-singles/tree/master/MagnumMath.hpp), depends on `CorradePair.h`
 
 There are also @ref corrade-singles "single-header libraries for Corrade APIs".
 

src/singles/MagnumMath.hpp

@@ -105,6 +105,7 @@
 
 #include "base.h"
 // {{includes}}
+
 #if (!defined(CORRADE_ASSERT) || !defined(CORRADE_CONSTEXPR_ASSERT) || !defined(CORRADE_INTERNAL_ASSERT_OUTPUT) || !defined(CORRADE_INTERNAL_ASSERT_UNREACHABLE)) && !defined(NDEBUG)
 #include <cassert>
 #endif
@@ -182,6 +183,11 @@
 #define MAGNUM_EXPORT
 #endif
 
+/* CorradePair.h is a dependency */
+#pragma ACME noexpand CorradePair.h
+#pragma ACME enable Corrade_Containers_Pair_h
+#include "CorradePair.h"
+
 /* A semi-verbatim copy of Utility/StlMath.h because otherwise the includes
    don't stay in the correct place. */
 #ifndef Corrade_Utility_StlMath_h