Fix builds in conda-forge, which doesn't have CLOCK_BOOTTIME

Fixes #8422

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/platform_util.c b/library/platform_util.c
index 09216ed..fdafa1f 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -265,7 +265,7 @@
     struct timespec tv;
     mbedtls_ms_time_t current_ms;
 
-#if defined(__linux__)
+#if defined(__linux__) && defined(CLOCK_BOOTTIME)
     ret = clock_gettime(CLOCK_BOOTTIME, &tv);
 #else
     ret = clock_gettime(CLOCK_MONOTONIC, &tv);