Fix prototype error with header on clang

Fix these errors reported by clang:
```
FAILED: [code=1] test/c99_header.p/test.c.o
clang -Itest/c99_header.p -Itest -I../test -I../include -fdiagnostics-color=always -DNDEBUG -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -Wpedantic -Werror -std=c99 -O3 -pedantic -MD -MQ test/c99_header.p/test.c.o -MF test/c99_header.p/test.c.o.d -o test/c99_header.p/test.c.o -c ../test/test.c
In file included from ../test/test.c:2:
../include/ffms.h:436:30: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
  436 | FFMS_API(int) FFMS_GetVersion();
      |                              ^
      |                               void
../include/ffms.h:437:31: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
  437 | FFMS_API(int) FFMS_GetLogLevel();
      |                               ^
      |                                void
```

Author: moi15moi

include/ffms.h

@@ -433,8 +433,8 @@ typedef int (FFMS_CC *TIndexCallback)(int64_t Current, int64_t Total, void *ICPr
 /* Most functions return 0 on success */
 /* Functions without error message output can be assumed to never fail in a graceful way */
 FFMS_API(void) FFMS_Init(int, int); /* Pass 0 to both arguments, kept to partially preserve abi */
-FFMS_API(int) FFMS_GetVersion();
-FFMS_API(int) FFMS_GetLogLevel();
+FFMS_API(int) FFMS_GetVersion(void);
+FFMS_API(int) FFMS_GetLogLevel(void);
 FFMS_API(void) FFMS_SetLogLevel(int Level);
 FFMS_API(FFMS_VideoSource *) FFMS_CreateVideoSource(const char *SourceFile, int Track, FFMS_Index *Index, int Threads, int SeekMode, FFMS_ErrorInfo *ErrorInfo);
 FFMS_API(FFMS_AudioSource *) FFMS_CreateAudioSource(const char *SourceFile, int Track, FFMS_Index *Index, int DelayMode, FFMS_ErrorInfo *ErrorInfo);

src/core/ffms.cpp

@@ -87,11 +87,11 @@ FFMS_API(void) FFMS_Init(int, int) {
     });
 }
 
-FFMS_API(int) FFMS_GetVersion() {
+FFMS_API(int) FFMS_GetVersion(void) {
     return FFMS_VERSION;
 }
 
-FFMS_API(int) FFMS_GetLogLevel() {
+FFMS_API(int) FFMS_GetLogLevel(void) {
     return av_log_get_level();
 }