build: Fix missing void parameters in C

Add missing void parameters to several functions. This prepares the code
to apply the -Wstrict-prototypes warning.

Bug: 329
Change-Id: Id5b152e9f8d77d07e99a6e781b3a3c4ef9bd08c0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/35560
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_log/basic_log_test_plain_c.c b/pw_log/basic_log_test_plain_c.c
index ead81f3..ee82374 100644
--- a/pw_log/basic_log_test_plain_c.c
+++ b/pw_log/basic_log_test_plain_c.c
@@ -28,11 +28,11 @@
 #error "This file must be compiled as plain C to verify C compilation works."
 #endif  // __cplusplus
 
-static void LoggingFromFunctionPlainC() { PW_LOG_INFO("From a function!"); }
+static void LoggingFromFunctionPlainC(void) { PW_LOG_INFO("From a function!"); }
 
 static void CustomFormatStringTest(void);
 
-void BasicLogTestPlainC() {
+void BasicLogTestPlainC(void) {
   int n = 3;
 
   // Debug level