raw: Allow mutable spans to be extracted

Change-Id: I4b88502f24c00475e36918b0bbe8658b585e0384
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/stdplus/raw.hpp b/src/stdplus/raw.hpp
index 59e308e..2d8f8ee 100644
--- a/src/stdplus/raw.hpp
+++ b/src/stdplus/raw.hpp
@@ -75,7 +75,7 @@
 #ifdef STDPLUS_SPAN_TYPE
 template <typename T, typename IntT,
           typename = std::enable_if_t<std::is_integral_v<IntT>>>
-T extract(span<const IntT>& data)
+T extract(span<IntT>& data)
 {
     T ret = copyFrom<T>(data);
     static_assert(sizeof(T) % sizeof(IntT) == 0);