dsp: firmware_update: Fix missing #include <span>

std::span started being used in
dec237bcdafe ("tests: firmware_update: Clean up some use of
reinterpret_cast<>()").

It is unclear why CI was OK, a failure is reported with gcc
13.2.0-23ubuntu4 (building externally, not OpenBMC tools).

gitlint-ignore: UC1, B1
Fixes: dec237bcdafe ("tests: firmware_update: Clean up some use of reinterpret_cast<>()")
Change-Id: If5bfbcc72ddcbd31764d84338cf9f915c9d467a0
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
diff --git a/tests/dsp/firmware_update.cpp b/tests/dsp/firmware_update.cpp
index db1a10d..878ba75 100644
--- a/tests/dsp/firmware_update.cpp
+++ b/tests/dsp/firmware_update.cpp
@@ -9,6 +9,7 @@
 #include <bitset>
 #include <cstdint>
 #include <cstring>
+#include <span>
 #include <string>
 #include <string_view>
 #include <vector>