[OpenMP] Fix a condition for KMP_OS_SOLARIS. (#71831)

Line 75 of `z_Linux_util.cpp` checks `#ifdef KMP_OS_SOLARIS` which is
always true regardless of the building platform because macro
`KMP_OS_SOLARIS` is always defined in line 23 of `kmp_platform.h`:
`define KMP_OS_SOLARIS 0`.

Author: xingxue-ibm

openmp/runtime/src/z_Linux_util.cpp

@@ -72,7 +72,7 @@ struct kmp_sys_timer {
   struct timespec start;
 };
 
-#ifdef KMP_OS_SOLARIS
+#if KMP_OS_SOLARIS
 // Convert timeval to timespec.
 #define TIMEVAL_TO_TIMESPEC(tv, ts)                                            \
   do {                                                                         \