add hello-world solution

exercises/OpenMP/Makefile

@@ -4,7 +4,7 @@ VPATH += solutions
 CC          = gcc
 CFLAGS      = -fopenmp
 LIBS        =
-BINS = hello pi pi_spmd_simple pi_spmd_optimized pi_spmd_atomic pi_loop
+BINS = hello pi hello_pra pi_spmd_simple pi_spmd_optimized pi_spmd_atomic pi_loop
 
 ifeq ($(UNAME), Darwin)
 	CFLAGS = -Xpreprocessor -fopenmp

exercises/OpenMP/solutions/hello_pra.c

+#include <omp.h>
+#include <stdio.h>
+
+int main() {
+  #pragma omp parallel
+  {
+    int ID = omp_get_thread_num();
+    printf("hello(%d)", ID);
+    printf("world(%d) \n", ID);
+  }
+  printf("After all threads finished");
+}