pw_kvs: Update float test to build for device

Update test for Put of floating point value to to use float rather than
double to avoid compile warning with [-Werror=float-conversion].

Change-Id: Ifd4b8551cfe30164dc04f23e5fb375c87442185c
diff --git a/pw_kvs/key_value_store_test.cc b/pw_kvs/key_value_store_test.cc
index 345a56b..81238a6 100644
--- a/pw_kvs/key_value_store_test.cc
+++ b/pw_kvs/key_value_store_test.cc
@@ -311,7 +311,7 @@
 
 TEST_F(EmptyInitializedKvs, PutAndGetByValue_NotConvertibleToSpan) {
   struct TestStruct {
-    double a;
+    float a;
     bool b;
   };
   const TestStruct input{-1234.5, true};