Fix build issue

/home/jenkins-op/workspace/openbmc-repository-ci/openbmc/intel-ipmi-oem/src/firmware-update.cpp:29:
ISO C++ forbids converting a string constant -Werror=write-string
10:57:23    29 | static constexpr char *secondaryFitImageStartAddr =

Tested: builds

Change-Id: I66dcb5932388cda513124b88dcaa58e31bc034e6
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/src/firmware-update.cpp b/src/firmware-update.cpp
index 95d01f7..99402e8 100644
--- a/src/firmware-update.cpp
+++ b/src/firmware-update.cpp
@@ -26,7 +26,7 @@
 #include <sdbusplus/timer.hpp>
 #include <sstream>
 
-static constexpr char *secondaryFitImageStartAddr = "22480000";
+static constexpr const char *secondaryFitImageStartAddr = "22480000";
 static uint8_t getActiveBootImage(void);
 static void register_netfn_firmware_functions() __attribute__((constructor));